红茶发酵时，酸度越来越大，多酚氧化酶在低于最适PH（）时，其活性越来越低
A、4
B、6.5
C、5.5
D、4.5
参考答案：C
It's so noisy. I'd really appreciate ______ if you'd turn the radio down.
[ ]
A. that
B. it
C. this
D. which
在SQL中，下列涉及空值的操作中错误的是______。
A) AGE IS NULLB) AGE IS NOT NULLC) AGE=NULLD) NOT(AGE IS NULL)
