—Where does your mother work?

—        she's working abroad.[ ]

A. At present

B. It's hard to say

C. Later on

D. No matter where

痰主要由（　　）等组成的，所以不要随地吐痰．

A．唾液、溶菌酶

B．黏液、细菌、灰尘

C．病菌、二氧化碳

D．灰尘、溶菌酶