某标准滴定溶液的浓度为0.5010moL·L-1，它的有效数字是（）。
A、5位
B、4位
C、3位
D、2位
参考答案：B
—When ______ you born, David?
—I ______ born in 1995. [ ]
A. was; were
B. were; was
C. were; were
D. was; was
______ we plant, _______ our city will be. [ ]
A. The more trees, the beautiful
B. The less trees, the more beautiful
C. The more trees, the more beautiful
D. The less trees, the beautiful
