下列说法正确的是（　　）

A．pH越小，溶液的酸性越强

B．pH小于5.6的雨水是酸雨

C．胃酸过多的病人在空腹时最好多喝一些柠檬汁

D．用pH试纸伸入某溶液中测定其pH

All of us thought _________ no use talking about some unpractical matters frequently.

A．that

B．it

C．this

D．which