It is chairman of the department ______ the teaching staff elected him.

单项选择题I prefer ______ there doing nothing.

单项选择题It is foolish ______ such a mistake.

单项选择题No sooner had we reached the top of the mountain ______ we all sat down to rest.

单项选择题Neither Mary nor her brother ______ good at singing.

单项选择题I don't think it advisable ______.