蠕形螨是（）

A.无致病作用的寄生虫

B.致病力较弱的寄生虫

C.致病力较强的寄生虫

D.机会致病寄生虫

E.偶然寄生虫

Last year, an appliance store sold an average (arithmetic mean) of 42 microwave ovens per month. In the first 10 months of this year, the store has sold an average (arithmetic mean) of only 20 microwaves per month. What was the average number of microwaves sold per month during the entire 22-month period

A．21

B．30

C．31

D．32

E．(E) 44