问题描述：
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
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
问题解答：
我来补答展开全文阅读