电源电压可直接影响紫外线灯辐射强度，当电压由220V降到200V时，紫外线辐射强度可降低多少（）

A.10%

B.20%

C.25%

D.30%

E.15%

—May I come in? I’m sorry I’m late. 

—Come in, please. But could you tell me         ?

A．how do you come to school

B．what were you doing then

C．who you talked with

D．why you are late