Answer:
Over the course of many years.
Explanation:
I'm not an expert, but I'd think that it would be over the course of many years. He didn't love her at first, so it wouldn't be slowly but consistently. However, he did know her for a long time, so it wouldn't be immediately and instantly. I think the pistol reference was only to confuse you to B, even though he didn't fall in love with her when he was 8.
The very first part “The students completed the math assignment” is not parallel with its other parts. Since the other sentences discuss either English or words/terms, it wouldn’t be a good idea to add the first sentence in this.