We covered --- an hour that day.

B) two miles and a half 为正确选项.
句子的意思是,“那天我们每小时走2英里半的路.”
“2英里半”英语是two miles and a half.或two and a half miles.这里miles要用复数,因为超过了1.而half的前面一定要用a,即 a half 才对.
此类短语还有很多：two hours and a half=two and a half hours“两个半小时”；four kilos and a half=four and a half kilos“四公斤半”.它们的用法都一样.