You were working too hard. You’d better keep a ________between work and relaxation.

A．promise

B．lead

C．balance

D．diary

检验一瓶长期露置在空气中的氢氧化钠是否变质，下列不能作为检验用的试剂是[ ]

A．澄清石灰水

B．氢氧化钡溶液

C．氯化钙溶液

D．酚酞试液