健康成人红骨髓约占骨髓总量的（）。
A.30%
B.40%
C.50%
D.60%
E.70%
参考答案：C
There are a lot of tourist spots in the city, _______ attracts millions of visitors every year. [ ]
A. which
B. that
C. in which
D. where
水冷壁吸收炉膛的高温火焰的辐射热，使水变成过热蒸汽（）
