(2012. Pudong New Area three mode) "a=3" is "straight line ax+2y+3a=0 and straight line 3x+ (A-1) y=a-7 parallel". A. Sufficient and unnecessary condition B. necessary and insufficient condition C. sufficient and necessary condition D. neither sufficient nor necessary condition

(2012. Pudong New Area three mode) "a=3" is "straight line ax+2y+3a=0 and straight line 3x+ (A-1) y=a-7 parallel". A. Sufficient and unnecessary condition B. necessary and insufficient condition C. sufficient and necessary condition D. neither sufficient nor necessary condition

When a = 3, the equations of two straight lines are 3x + 2Y + 6 = 0 and 3x + 2Y + 4 = 0, then the two lines are parallel. On the contrary, when two lines are parallel, there is - A2 = 31-a but - a ≠ a-7a-1, that is, a = 3 or a = - 2, then there is not necessarily a = 3, so "a = 3" is a sufficient and unnecessary condition for "the line ax + 2Y + 2A = 0 and the line 3x + (A-1) Y-A + 7 = 0 are parallel"