科学施用化肥是农业增产的重要手段。下列属于氮肥的是[ ]
A.K2CO3
B.Ca(H2PO4)2
C.NH4HCO3
D.K2SO4
C
A new library is being built _______ there was a playground .
A.that
B.in which
C.where
D.when
电路如图所示,Us为独立电压源,若外电路不变,仅电阻R变化时,将会引起下述哪种变化?()
A.端电压U的变化
B.输出电流I的变化
C.电阻R支路电流的变化
D.上述三者同时变化