在“性状分离比的模拟”实验中，某同学连续抓取三次，小球的组合都是Dd，则他第4次抓取Dd
的概率是                                                                            

A: Excuse me, are you Sam?

B: Yes, I am.____

A: My name is Bob.

B:____

A: Nice to meet you, too.____

B: No, I'm not.____ Where are you from?

A: I'm from Beijing.____

B: Thank you.