Failure can make a positive contribution to your life______you leam to use it．
| A．since | B．unless | C．until | D．once |
Mary is the only one of the students in our class who ____ to the Great Wall so far.
| A．is | B．has been | C．are | D．had been |
Two years ago, the 9-year-old girl set up ___ organization named Angel to help ____ homeless.
| A．the; | B．; the | C．an; a | D．an; the |
For miles around me there was nothing but a desert, without a single plant or tree ______.
| A．on earth | B．in sight | C．at a distance | D．in place |
More and more people choose to shop in a supermarket as it offers a great ____ of goods.
| A．expression | B．mixture | C．variety | D．combination |
So difficult ______ it to live in an English-speaking country that I determined to learn English.
| A．did I feel | B．I have felt | C．have I felt | D．I did feel |