—Because she_________ a cold.
A:is, is
B:isn't, is
C:isn't, has
函数fx=mx2-2x+1的定义域为R,则实数m的取值范围是( )
—— did you play the piano today?
——For five hours.
某兴趣小组同学在实验室将一定量的稀硫酸加入到盛有氢氧化钠溶液的烧杯中,未看到明显的现象.部分同学产生疑问:酸和碱到底能否发生反应?