题目

Some people worry about being the target of laughter. These people are frightened. They suffer from an emotional disorder called gelotophobia. That long name comes from the Greek language. The word Gelos means laugh, while phobos means fear. Victor Rubio is an expert on human behavior at the Autonomous University of Madrid. He says people laugh at others for many different reasons. He says being laughed at causes a fear response in the victim. That fear leads the victim to avoid social situations. Sadly, gelotophobia limits the way they lead their lives. Victor Rubio was among researchers in18．已知随机变量X的分布列为$P（X=k）=\frac{a}{2^k}，k=1，2，…10$，则P（2＜X≤4）=（　　）A．$\frac{16}{341}$B．$\frac{32}{341}$C．$\frac{64}{341}$D．$\frac{128}{341}$