— Have you been to the Great Wall? —Perhaps not in my memory. ________, it might have been during my early childhood.

Smoking is so harmful to people’s health that it kills __________ people each year than automobile accidents. 

Only when _____ possible to settle the problem how to get the machine repaired.

The global warming is more and more serious, for which human beings ______．

You’d better come to my home tomorrow, because ___ to show you some of my best stamps.