从A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
例：We——last night, but we went to the concert instead
A.must have studied B. might study
C .0should have studied D. would study
答案是C。
21. Although it ________ as hard as ____stone, it easily melts when ______.
A. feels…\...heated B. feels…a...heated
C. is felt…a...heated D. feels…\...heating
