Calling all hands on Capitol Hill, and in the lobbying canyons of K Street. I am about to hit you with an impossible assignment -- so impossible that you won't be able to resist.

What follows is the toughest quiz about the inhabitants of the House and the Senate I have ever encountered. I spent three hours poring over it, and got only two of the 20 questions right. I could make guesses (wrong guesses) at no more than three others. To the remaining 15 questions, I murmured, "Only a computer program would know that."

In fact, a computer program does. Its name is Hill Search, and it belongs to Parkhurst-Spence Inc., a Washington legislative consulting company.

When PSI began trying to market Hill Search a few months ago, it devised The World's Hardest Hill Quiz to demonstrate how well Hill Search finds and remembers details about members of Congress. Hill Search finished the quiz in five minutes and 57 seconds. No human being ever has.

Can you? Here are the 20 questions that try men's and women's souls. If you already subscribe to Hill Search, no cheating! Answers in a few days:

1) Which member of the House serves on the largest number of committees and subcommittees? How many of each?

2) How many members of Congress are former governors? Who are they?

3) How many members were born on Jan. 1? Who are they?

4) Name a senator who was formerly a farmer.

5) Which member of Congress used to be a veterinarian?

6) Who is the youngest member of Congress?

7) Who is the oldest?

8) Which Republican is the oldest member in terms of seniority? Which Democrat?

9) How many Hispanics are members of Congress? Name them.

10) How many women serve in Congress? Name them.

11) In how many states are both senators former members of the House? Which states?

12) How many members of Congress are former Hill staffers? Names, please.

13) The spouse of which member is a college professor? What is the spouse's first name?

14) How many members have spouses who are attorneys? Name the members and the spouses.

15) Who is former representative Floyd Fithian's present employer?

16) How many states have 15 or more congressional districts? Which states are they?

17) How many members of Congress live at the Watergate? Who are they?

18) Which state delegation has the largest number of women in Congress? What are their names and congressional districts?

19) Which three female members of Congress tie for the most conservative rating on social-issue votes?

20) Astrologers claim that Leos like to be center-stage. How many members of the Senate were born under this sign? Who are they?

Happy squirming!

Is that an editorial, Diamond Cab of Virginia? Or is it an egregious misspelling?

Gail Charnoff of Fairfax is still looking at a receipt from Diamond, and wondering. The categories on the receipt are: Run Number, From, To, Date, Time, Signature, Cab Number -- and "Fair."

As Gail so neatly puts it, "Perhaps Diamond Cab thinks the fare is fair. But this fair the one on the receipt is not fare!"

H. Milton Chase of Arlington reports that some girls prefer to remain single, but most would rather knot.

And in the high school reunion department . . . .

Largo High class of '76 -- Sept. 6. Further information: 249-3352.

Montgomery Blair '66 -- June 20-21. Call 465-7322 or 587-2700.

Woodrow Wilson 50th anniversary -- March 1-8. Call Dorothy Brown, 282-0120.

Sacred Heart Academy, all classes between 1960 and 1974 -- Organizing. Call Cristina Giambi (622-0425).

Cardozo '43 -- Organizing. Call Richard Farrow (396-7093) or Mildred Scott Strawder (423-3324).

Langley '76 -- Organizing. Call Susan Wulf (525-6543) or Maripat Hinders Goggin (631-6006).

Woodbridge '76 -- Oct. 4. Call Kathy Colassard (491-1810).

Roosevelt '36 -- Organizing. Write to Reunion Committee, P.O. Box 155, Oxon Hill, Md., 20745.

Eastern '46 and '47 -- Organizing. Call Maria Fontana Corvelli (370-6339) or Doris Jaeger McCawley (736-8885).