We can learn many people's names at the _____ this book. [ ]

A．begin of

B．beginning of

C．begining at

可一次将AgNO₃、Na₂CO₃、MgCl₂三种无色溶液区别开来的试剂是（　　）

A．HNO₃溶液

B．NaCl溶液

C．Na₂SO₄溶液

D．稀盐酸