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？

请教：为什么(B)选项不正确？