—Could you please tell me how to get on well with my schoolmates?

—________

A．Try to be confident and work hard.

B．Be helpful and friendly.

C．Choose the white dress. You look nice in white.

D．You should have a lot of money.

为了节约用水，某自来水公司采取以下收费方法：每户每月用水不超过10吨，则每吨水收费1元，如果每户每月用水超过10吨，超过部分按每吨2.5元收费，现在李老师家里2月份用水a吨（a＞10吨）．

（1）请用代数式表示李老师2月份应交水费多少元？

（2）如果a=16，那么李老师2月份应交水费多少元？