常温下，下列关于pH值的说法正确的是

A．在pH＝12的溶液中，由水提供的c(OH^－)一定是10^－12mol/L。

B．把pH＝2与pH＝12的酸、碱溶液等体积混合后，所得溶液的pH值为7

C．pH＝6与pH＝4的两盐酸等体和混合后，所得溶液中c(H^＋)最接近0.5×(10^－8＋

10^－10)mol/L

D．在稀氨水里加入NH₄Cl晶体后，pH值会降低，若再向此溶液中加入少量稀盐酸或少量氢氧化钠溶液后，溶液的pH值都几乎不变

Donna is a mountain biking enthusiast. One Saturday, she spent the morning biking up an uphill trail at an average speed of 20 kilometers per hour, and then returned by the same route in the afternoon at an average speed of 25 kilometers per hour. If the downhill trip in the afternoon took 3/4 of an hour less than the uphill trek in the morning, how many kilometers did Donna ride each way ？（）

A. 50

B. 55

C. 65

D. 70

E. 75