(1) The concerns about sending people to Mars is definitely worth considering. (2) For one thing, the mission will cost billions of dollars. (3) This would be a huge additional cost at a time when national debts continue to skyrocket. (4) Also, that money could help fix problems here on Earth.

Which sentence needs revision to fix a grammar error?

Sentence 1 should be revised to change “is” to “are.”
Sentence 2 should be revised to change “will cost” to “costs.”
Sentence 3 should be revised to change “continue” to “continued.”
Sentence 4 should be revised to change “could help” to “helps.”