求助几道英语题

1---arriving in Tibet, the soldiers at once joind them in their work AAt BOn CThe moment DIn
2When spring comes, everywhere appears flowers. His speech isn't easy--- Ato be understood Bunderstood Cunderstanding Dto understand
3Would you be kind enoughto take a photo of me ----under the tree?Astand Bstanding Cto stand D stood
4Take an umbrella to--you from the rain,which may --you from taking a walk.Aprotect;stop B protect;protect Cprevent;protect
Dprevent;keep
请讲原理，谢谢

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