单选题 Tony was in plain clothes, watching for a______character at London Airport all night.
【正确答案】 A
【答案解析】解析：A项suspicious“令人怀疑的，可疑的”；B项suspect是名词，意为“嫌疑犯”；C项susceptible“易受影响的”，后接介词to；D项doubtful“疑惑的，怀疑的”，常与about、of连用，一般不修饰名词。