0.27,B is so,so.In fact,I can't do fast as young man

You are good student,general question is at least two students have same,...,In this case,we can do like this:
1-P(30,5)/30^5=0.296266667.
1...=P(30,5)/30^5=0.70373333
2...=P(30,4)*C(5,2)/30^5=0.270666667
3...=P(30,3)*C(5,3)/30^5=0.01002469
4...=P(30,2)*C(5,4)/30^5=0.000179012
5...=P(30,1)/30^5=1.457*10^(-43)

.7037333+.270666667+.01002469+...=1. OK.