-What subject is difficult for Mary? ( )
- __________________
A:Her friend Peter.
B:In Scotland.
C:Maths.
我们常听到以下说法:①生理盐水 ②高原缺氧 ③煤气中毒 ④胃酸过多.请从H,C,O,Na,Cl中选择合适的元素,书写化学式: