The little boy is dirty from head to foot because he ________ in the mud all morning.

A．has played

B．is playing

C．has been playing

D．was playing

若将两个电源的U-I图象画在同一直角坐标系上，如图所示，由图象可得（　　）

A．电动势E₁＞E₂，内阻r₁＜r₂

B．电动势E₁=E₂，内阻r₁＞r₂

C．发生相同的电流变化，电源1的路端电压变化较大

D．发生相同的电流变化，电源2的路端电压变化较大