应该选择C
a lot more fun 意思是：有趣得多
例如：1.That must be a lot more fun than taking a bus.
那比起乘公共汽车要有趣得多.
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2.By boat must be a lot more fun than taking a car.
乘船肯定比乘车有意思得多.
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3.Of course,when the trail is muddy,it is also a lot more fun.
当然,当踪迹是泥泞的时候,它也是很多较多的乐趣.
而选项D,fun 是名词不能这么使用