A:should have failed
B:might have failed
C:couldn’t have failed
D:mustn’t have failed
如图,直线a∥b,∠1=85°,∠2=35°,则∠3=( )
如图所示的闭合电路中,电源电动势E=12V,内阻r=1Ω,灯泡A标有“6V,3W”,灯泡B标有“4V,4W”.当开关S闭合时A、B两灯均正常发光.求:R1与R2的阻值分别为多少?
绿色开花植物在完成受精作用后,所形成的受精卵将发育成( )