【正确答案】 B

【答案解析】[解析] 画线词组at once为动词词组，意为“马上”。A项soon意为“很快”；例：We'll have lunch soon.我们很快就要吃午饭了。B项immediately意为，“马上”；例：It's getting late. We must go back home immediately.越来越晚了，我们得马上回家。C项now意为“现在”；例：If we leave now we'll be there before dark.如果我们现在就离开，在天黑前就到那里了。D项early“早”；例：Mary gets up very early.玛丽起得很早。故本题选B。