欲配制pH=5的缓冲溶液,应选用下列( )共轭酸碱对。<br/> A、NH2OH2+-NH2OH(NH2OH的pKb=3.38)<br/> B、HAc-Ac-(HAc的pKb=4.74)<br/> C、NH4+-NH3·H2O(NH3·H2O的pKb=4.74)<br/> D、HCOOH-HCOO-(HCOOH的pKa=3.74)