AC is 1.

Then AG=0.5=GC.

AM = MB = 0.5(AG+GB)= 0.5(0.5+x)= 0.25+0.5x

BN = NC = 0.5(GC-GB)= 0.5(0.5-X)= 0.25-0.5X

In this way, it can be concluded that MN=MB+BN=0.5=AG=GC is correct.

Gn = Gb+BN = X+0.25-0.5x = 0.25+0.5x = 0.5 (Ag+GB) Correct.

Mg = MB = GB = 0.25+0.5x-X = 0.25+0.5x = 0.5 (Ag-GB) is correct.

MN=MB+BN=0.5=AG=BC D Wrong.

This is concrete and simple. You can directly observe options a and d.

Obviously, GC is not equal to 1/2(AC+GB), so there must be something wrong with AD. MN is the midpoint between AB and BC, and there must be MN that is half the length of AC, so it is easy to draw the conclusion that D is wrong.