______ life began on earth is one of the biggest puzzles that scientist found hard ______.
| A．How; solving | B．When; to be solved |
| C．How; to solve | D．Which; to solve |
-–Mary knows the city quite well.
--She______ well have been there before.
| A．need | B．could | C．should | D．may |
The past two years ______a sharp rise in oil price, which adds to the burden of auto transportation industry.
| A．has witnessed | B．witnessed |
| C．is witnessing | D．witnesses |
The moment ______Leo will never forget is ______he joined the Air Force
| A．when; that | B．that; that | C．that; when | D．when; / |
I prefer to work here because salaries are higher here than ______in my country.
| A．that | B．the ones | C．those | D．one |
The girl ___on the ground ___to me that she had ____the book on the shelf.
| A．lying; lay; laid | B．lay; lied; laid |
| C．lying; lied; laid | D．lie; lied; lay |