Problem: How does the sentence end?

Rachel gave a reward to Monica for doing a great job on the account, since 
Answer: Rachel was a good boss.

Problem: How does the sentence end?

Mary wanted to keep Tanya from entering the house, but 
Answer: Mary got to the door after her.

Problem: How does the sentence end?

Carrie asked Christine to braid her hair for the party because 
Answer: Carrie was not skilled in braiding.

Problem: How does the sentence end?

Matthew knew that their ring contained real diamonds but Derrick did not because 
Answer:
Matthew was an expert on gemstones.