Don’t stand by when you come across have difficulty in supporting their family because of

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A. Who
(答案→)B. Whoever
C. Whichever
D. Whomever
解析：句意：当你遇到因为重病难以养家糊口的人不要袖手旁观。这里e across后面是宾语从句，宾语从句中缺少主语用whoever引导宾语从句，相当于anyone who，whomever在从句中做宾语，whichever“无论哪个”，who引导宾语从句的意思是“谁”，所以选B。