Do you know how often he ----（锻炼）?He is so strong.exercise用加

要加单三形式,答案为：exercises
从句中he是主语,是第三人称单数,且从句的时态明显是一般现在时,所以答案用exercises没错.
再问： The twins have no d_____from each other.
再答： difference 区别/差别/不同 当然你要用differences也可以。