Find the one mistake (A, B, C or D) in these sentences and then correct them

A. Because of

B. has been

C. seriously ill

D. can't go

A. Though 

B. not good at

C. palying

D. to win

A. is known

B. these questions

C. so difficult

D. can’t answer

A. This water

B. warm enough

C. young boys

D. to swim

A. unless

B. had had

C. enough money

D. would have bought