The flu is believed _____ by viruses that like to reproduce in the cells inside the human nose and throat. [ ]

A. causing

B. being caused

C. to be caused

D. be have caused

在锅炉施工过程中，煮炉的目的是除掉锅炉中的油污和铁锈等，不能用来煮炉的药品是（）

A.氯氧化钠

B.碳酸钠

C.硫酸二钠

D.硫酸钙