Do you often hear John _____ in his room?[ ]
A. sing
B. singing
C. sings
D. to sing
答案:A
设f(x)=sinx,则f’’(x)=______.
植物生长需要养分.某植物由于缺氮出现了生长发育迟缓或停滞;又由于缺钾出现了易倒伏和抗病虫害能力减弱的状况.这时应施用的一种化肥是( )
A.KCl
B.NH4Cl
C.Ca(H2PO4)2
D.KNO3