英语完形填空,请问21空为什么选cSnowisasubjectofgreatinteresttoweatherexperts.Theysometimeshavedifficulty21where,whenorhowmuchsnowwillfall.Onereasonisthatheavyamountsofsnowfallinsurprisinglysmallare

　　英语完形填空,请问21空为什么选c

　　Snowisasubjectofgreatinteresttoweatherexperts.Theysometimeshavedifficulty21where,whenorhowmuchsnowwillfall.Onereasonisthatheavyamountsofsnowfallinsurprisinglysmallareas.Anotherreasonisthatasmallchangeintemperaturecanmeanthe22betweensnowandrain.

　　21.A)estimates

　　B)estimate

　　C)estimating

　　D)estimated