@YuriTokoro
1. You can say 'while' instead of 'during which'. There's no real difference. Its just quicker to say.
2. 'One variety of' means a certain group out of multiple groups. For example: Car makers Toyota, Nissan and Honda all make various models of car. If you were looking for 'one variety of' Nissan, you'd be looking for a certain kind of Nissan. Whereas if you are looking for 'various' Nissan's, you'd be looking at every car Nissan makes. A poor example maybe, but it makes my point. As for the second question in 2, as your looking for 'one variety of' crop, you use the singular version, instead of the plural.
3. As for this question, I'm afraid I'm going to have to let someone else answer that one for you, as I have no idea why. lol
Anyway, I hope my answers have helped
