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下列四组线段中,能构成比例线段的是( )
A.1cm,2cm,3cm,4cm B.4cm,5cm,6cm,7cm
C.1cm,m,2m,4cm D.2cm,2cm,cm,12cm
下列三条线段不能构成直角三角形的是( )
A.5cm,13cm,12cm B.2cm,3cm,cm
C.4cm,7cm,5cm D.1cm,cm,cm