Strong sandstorms are often _____ you cannot see the sun.

We can hardly believe what she says because she is ___changing her mind.

“Perhaps we need to send for Dr. Smith to see what we can do about it,” Father suggested ________ to his neighbor who had come to discuss the problem.

My uncle's house in the downtown area is much smaller than ours，but it is twice________expensive.

Scientists will have to come_______ new methods of increasing the world’s food supply.