--- I'm sorry．That wasn't of much help．

---Oh,______．As a matter of fact,it was most helpful．[     ]

A. thanks anyway        

B. it doesn't matter        

C. of course not        

D. sure it was

遗传物质脱氧核糖核酸（DNA）的分子直径为0.0000002cm，用科学记数法表示为（    ）cm。