题干
–Where are the students? How could they keep us waiting for so long?
–They started an hour ago and they ____ be here in fifteen minutes, I think.
–They started an hour ago and they ____ be here in fifteen minutes, I think.
A：ca
B：would
C：should
D：must
答案(点此获取答案解析）
C
的图象上各点的横坐标伸长到原来的3倍，再向右平移
个单位，得到的函数的一个对称中心是（ ）