about the missing boy’s safety, the police stepped up their search.
A. Concerning
B. Concerned
C. Being concerned
D. Concern
网友回答
A. Concerning
(答案→)B. Concerned
C. Being concerned
D. Concern
解析：考查concern用法。Concerning关于；be concerned about担心…；本题是形容词短语做原因状语。句意：因为担心失踪孩子的安全，警察们加快了搜查的进度。