脱硫剂应选择冶金反应性能良好的（）石灰，避免采用过烧和生烧的石灰。
参考答案：活性
—_____ the population of the U.S.A. in 2005?
—It _____ about 296 million.
A.What is; is B.What was; was
C.How many is; was D.How many was; is
