Dr Frank failed many times but hefinally _______ to find a successful way to solve the problem

The split between humans and living apes isthought by some scholars ____ 15 to 20 million years ago.

Having never met such a ______ situation, Mr. Green felt ______ and didn’t know what to do.

How many more decades will have to pass ____ scientists succeed in providing a cure for cancer?

Can I smoke here?—No, at no time　　　in the library.