Two of the main thoughts in this passage are (1) preparation of forms for the concrete must be thorough, and (2) forms must be allowed to remain on: long enough. The third main idea is ____.

A、taking off forms beforehand will probably cause a crack in the body of the work
B、trying to make changes after concrete has been poured is not recommended
C、mixing concrete properly will make it very hard and strong
D、using as little water as possible is recommended