关于某些名词的单复数问题

1. spacecraft

It may be another fifteen years before spacecraft from Earth again venture to Mars, a planet now known to be cold, dry, and probably lifeless.

(A) again venture to Mars, a planet now known to be

(B) venture to Mars again, a planet now known for being

(C) will venture to Mars again, a planet now known as being

(D) venture again to Mars, a planet that is known now to be（A）

(E) will again venture to Mars, a planet known now as being

正确答案中用的是venture,而不是ventures来作spacecraft的谓语。大全注释上说这是因为spacecraft是复数，对于这一点我有些怀疑，请问有谁可以找出这种说法的依据吗？

2. staff

这个词Longman中给出的例句是这样的

The school's staff is/are excellent.

请问staff到底是单数还是复数形式呢？如果考到代词指代的话，应该用it还是they来指代呢？

Words such as Spacecraft and aircraft are uncountable. You can have 3 airplanes (or aeroplanes if you are Brittish) but you would say "several aircraft/spacecraft".
http://www.englishforums.com/English/SingularPluralWords/3/bmgg/Post.htm

Plural of "Spacecraft"

Which is the plural form of "spacecraft": "spacecrafts" or "spacecraft"?

I use "spacecraft" and Webster's Dictionary agrees with me.

Dr. Eric Christian
(May 2002)

http://helios.gsfc.nasa.gov/qa_mis.html#plural

十分感谢joe11。

'Words such as Spacecraft and aircraft are uncountable.'

但是我认为这句话表达有误，因为照下面的说法，spacecraft并非是不可数名词，而只是单复同形。另外那道题目好象也暗示了这个意思，因为它是将这里的spacecraft以可数名词复数形式处理的。