语法问题，请各路大侠指教

1. Hail is formed when a drop of rain is carried by an updraft to an altitude where_____ to freeze it.

A. is the air cold enough

B. the air cold enough

C. the cold enough air

D. the air is cold enough

答案C, 我觉得应该是D, where 引导的壮语从句不是应该是完整的一句话吗？

2. In the arily 1900's Roy Harris created and pormoted a distinctly American style of classical

music and greatly influenced a number of composer in the Unitied States.

答案 B, 请问D—a number of composer 为什么不用复数？

3. For an advertisement to be effective, its production and placement must to be based on a

knowledge of human nature and a skilled used of the media.

答案C， 请问D 为什么是skilled 而不是skill

1.我也认为答案有错，应该是d,不过where引导的不是壮语从句，是定语从句。

2.我认为是印刷错误，应该有s。

3.同意你的看法，skill

1.怎么对的答案呀？正确答案本来就是D。

2.正确答案应该是D。composer--->composition

3.哪里找的题呀？D选项打印错了，应该是skilled use。

到底2，3应该是怎样的？

distinctly对吗？

1.D

2.composers