Two men and two boys are planning to cross a river using a small boat that can only hold either one man or two boys. What is the least possible number of times the boat needs to cross the river in order to bring all four of them to the other side? 
选项为:A:$$3$$  B:$$6$$  C:$9$  D:$$11$$  E:$$13$$  请回答其中一个选项.