2 foxes and 2 rabbits want to cross a river. There is a boat that can hold 2 animals. When the number of fox is less than the number of rabbits, the rabbits fight with the foxes. For 2 foxes and 2 rabbits to cross the river safely, at least how many times does the boat need to cross the river? (count the round trip as twice and every trip need one animal row the boat)~\uline{~~~~~~~~~~}~． 
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Assume the foxes as A, and the rabbits as B. The way they cross the river as following:  $$AA\xrightarrow{BB}$$  $$AA\xleftarrow{B}B$$  $$B\xrightarrow{AA}B$$  $$B\xleftarrow{B}AA$$  $$\xrightarrow{BB}AA$$ 