21. The chief foods eaten in any country depend largely on _____ best in its climates and soil.

A. it grows   B. what grows    C. it does grow   D. what does it grow

22. A 3G mobile phone, _____to be most useful and fashionable, is popular with some teenagers.

A. consider          B. considering        C. considered        D. to be considered

23. We thought there were 35 students in the dining hall, ___, in fact, there were 40.

A. while             B. whether           C. what             D. which