时态正确呀
在宾语从句中如果是客观事实要用一般现在时,所以：The boy asked his mother if water freezes at zero.男孩问他的母亲,水在零度是否结冰.water freezes at zero.属于客观事实,尽管主句是一般过去时（ asked),从句还是用一般现在时(freezes)
例如：The teacher told us the earth travels around the sun.