7. 在等距量尺下，若a大於b、b大於c、c大於d，則下列哪一數學式可表示其關係？
(A) a = b = c = d
(B) ( a – b ) = ( c – b ) = ( d – c )
(C) a = 2b = 3c = 4d
(D) ( a – b ) = ( b – c ) = ( c – d )