E(x)=0.5，D(x)=0.5
E(x)=0.5，D(x)=0.25
E(x)=2，D(x)=4
E(x)=2，D(x)=2
考点：泊松分布E(X)=D(X)=λ，其中λ为参数故本题，E(X)=D(X)=2
