同学,很高兴回答你的问题
正确答案：选C.is
即：Climbing hills is of great help to our health.
翻译：爬山对我们的健康是有帮助的（有好处）.
解析：be of great + n.= be very + adj
所以这句话的同义句是；
Climbing hills is very helpful to our health.
of great help 是作表语
有不明白的请继续追问,