I picked up D first over C, then after examining closely, I found :
in D,
A nuclear weapon detonated above the atmosphere would not generate a blast, shock, or heat, but rather an intense electromagnetic pulse disrupting, like a lightning bolt, all unshielded electric lines and electronic equipment.
=> A nuclear weapon detonated above the atmosphere would not [this ] but [that].
So IMO,for D to be correct it should be , ......would generate not..[this] but [that].
in C,
A nuclear weapon detonated above the atmosphere would generate no blast, shock, or heat, but rather an intense electromagnetic pulse disrupting, like a lightning bolt, all unshielded electric lines and electronic equipment
=> A nuclear weapon detonated above the atmosphere would generate no [this] but [that].
It is just my understanding...
Anyone, please correct me if I am wrong.