It is these poisonous products ______can cause the symptom of the flu, such as the headache and aching muscles.

____ important to learn English well.

It is exactly _____we behave _____has changed the world.  

It was about 600 years ago __________ the first clock was made.

—Was it in April, 1970         China succeeded              sending its first satellite into space? —Yes, that’s right.