-- Did you hear anything ____ (usual) in the next door?
-- No, I was chatting online with friends.
一个生态系统包含的生物种类越丰富,则该生态系统越稳定.
如图所示,电源电压保持不变,当电键S闭合滑动变阻器的滑片P向右滑动时,下列说法正确的是 ( )