I think B. We know there is just enough cake for 30 people. 30 slices for 30 people.
A) Tell us nothing about how many total ate cake. Could be 15 ate cake or 20. Doesn't answer yes/no to did everyone eat cake.
B) Suff. Tell us that somewhere between 0 - 19 people ate more than 1 slice of cake. Even if everyone tried to eat a piece of cake, they couldn't now that at least 1 person had a slice. Since someone ate two slices, someone missed having a slice. So no, not everyone ate cake. Some fatty ate two!
Is it not possible that 0 people ate more than one slice of cake? 0 is less (fewer) than 20.