透镜的焦点用字母 ______表示，焦距用字母 ______表示．
透镜的焦点用字母“F”表示，左右各一；焦距用字母“f”表示．
故答案为：F；f．
The old man, ________ abroad for twenty years, is on the way back to his motherland.
A．to work
B．working
C．to have worked
D．having worked
Jim _________ a green pen.[ ]
A. has
B. had
C. have
