Last summer I took a course on _______.
A:how to make dresses
B:how dresses be made
C:how to be made dresses
D:how dresses to be made
设p是椭圆x225+y216=1上的点.若F1,F2是椭圆的两个焦点,则PF1+PF2等于( )
写出化学符号或化学符号所表示的意义:
下列词语中没有错别字的一组是( )