Does he like tea or coffee—Sorry. I don’t know ______ he prefers.

The thought ______ he might fail in the exam worried him.

It __be the vocabulary that caused you the problem in the exercise because you know a lot of words.

What did Lily tell you just now—-She asked me _______ others her secret.

Astronomers and scientists think that a black hole is a region of space ______ which matter has fallen and _______ which nothing can escape.