堪称古典时代的百科全书的是（）
A.《管子》
B.《礼记》
C.《周礼》
D.《尚书》
参考答案：C
重铬酸钾在酸性环境下被氧化成( )。
A．Cr7+
B．Cr1+
C．Cr5+
D．Cr3+
There _____ many people in the park last Sunday.[ ]
A. had
B. was
C. were
D. are
