I once lived in a house _______ roof has fallen in.

Susan thought ____not easy to answer all these questions.

He gave me two answers_________the question,but________of them is right.

Who will _______ the baby when your aunt is away?—Nobody but me.

The Gulf of Mexico (墨西哥湾) is being polluted seriously. That has affected _______ several hundred kinds of sea animals there. [ ]