Problem: How does the sentence end?

Her child had dropped the birthday cake onto the floor, and she was upset because the 
Answer: cake is ruined.

Problem: How does the sentence end?

She preferred to practice music on guitar instead of piano, because the 
Answer: piano is harder for her.

Problem: How does the sentence end?

Nelson asked Robert what kind of rifle was in the case but 
Answer: Robert had forgotten.

Problem: How does the sentence end?

He boiled water in the pot a lot more than oil, because the 
Answer:
water reacted well to the pot's composition.