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下列各式计算正确的有( )个
①(a-2b)(3a+b)=3a2-5ab-2b2
②(2x+1)(2x-1)=4x2-x-1
③(x-y)(x+y)=x2-y2
④(2+x)(3x-6)=3x2-12
A.1
B.2
C.3
D.4