—_______I ______ in the homework before lunch?

—Yes. Please hand it in as soon as possible. [ ]

A. Ought; hand  

B. Ought; to hand    

C. Have; hand        

D. Must; to hand

酚酞在酸性溶液中主要以下列结构I形式存在，在碱性溶液中主要以结构Ⅱ存在，试解释为什么酚酞在酸性溶液中呈无色而在碱性溶液中为红色。