It is required that such insulator(绝热体) ______ a heat resistant

It is required that such insulator(绝热体) ______ a heat resistant material.
A、must be made of
B、should be made of
C、will be made of
D、would be made of
【正确答案】：B
【题目解析】：在“It is required that…”句型中that从句中要用虚拟语气，其表示方法是should do。类似的还有“It is desired that…”，“It is suggested that…”，“It is ordered that…”等。