英语完形填空,请问21空为什么选c

英语完形填空,请问21空为什么选c
Snow is a subject of great interest to weather experts. They sometimes have difficulty 21 where, when or how much snow will fall. One reason is that heavy amounts of snow fall in surprisingly small areas. Another reason is that a small change in temperature can mean the 22 between snow and rain.
21. A) estimates
B) estimate
C) estimating
D) estimated

have difficulty (in) doing sth
做某事有困难
再问： 可以帮我翻译一下这个句子意思吗，谢谢啦
再答： 他们（天气预报专家）有时难以估算的地点、时间，雪会下降多少