混凝土碳化
参考答案：
混凝土的碳化是指环境中的CO2，与水泥水化产生的Ca（OH）2作用，生成碳酸钙和水，从而使混凝土的碱度降低的现象。
—Tom is good at swimming.
—Yes. He swam across _______ river last Friday.[ ]
A. a 11-meter-wide
B. an 11-meters-wide
C. an 110-meter-wide
D. a 110-meter- wide
Jenny was born_____ America. [ ]
A. in
B. on
C. at
