以下说法正确的是                                                 [ ]

A．将pH=4的醋酸溶液稀释后,H⁺浓度增大

B．常温下，0.01LpH=10的NaOH溶液加水稀释到10L,此时溶液的PH=7

C．改变条件使醋酸的电离平衡向右移动,醋酸的电离度一定增大

D．常温下0.1mol·L^－1醋酸溶液的pH＝a，加入适量的醋酸钠固体能使溶液pH＝(a＋1)

The days are gone________you can breathe fresh，unpolluted air or drink clean, unpolluted water in the countryside.

A．when　

B．that

C．where　

D．which