B, as there is no point in a comma there. There is already an and in the sentence which is a conjunction.
1. The whole class laughed loudly at the story.
complete subject: the whole class
complete predicate: laughed loudly at the story
2. The whole class laughed loudly at the story.
simple subject: class
simple predicate: laughed
3. Twelve hours passed without a word from any of the group.
simple subject: hours
simple predicate: passed
4. The workmen repaired the dam.
complete subject: the workmen
complete predicate: repaired the dam
