12.
Your Answer is
Correct Answer is G
Explanation
This sentence is two complete sentences, which need to be connected by conjunctions. The G item is a major-slave conjunction, which can be placed at the beginning of the sentence to connect the two sentences, so it is correct.
F/H Item: It is an adverb and cannot connect two sentences.
Item J: It is a coordinating conjunction, connecting two sentences can only be placed in the middle of the two sentences, so it is wrong.