问题
选择题
Susan finally became a popular singer after she practiced ______ for years. [ ]
A. sing
B. to sing
C. singing
D. sang
答案
答案：C
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Susan finally became a popular singer after she practiced ______ for years. [ ]
A. sing
B. to sing
C. singing
D. sang
答案：C
4H2O + 3CO2），现在准备改使用天然气，如果在使用的灶炉不换的情况下，应该怎样调节灶炉的进燃气口和进空气口才更合理。