20. The coefficient of $x ^ { 2 }$ in the expansion of $\left( 4 - x ^ { 2 } \right) \left[ \left( 1 + 2 x + 3 x ^ { 2 } \right) ^ { 6 } - \left( 1 + 4 x ^ { 3 } \right) ^ { 5 } \right]$ is
A 28
B 72
C 78
D 192
E 240
F 310
G 312
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