[求助]一道网络教材中的语法题

一道网络教材中的语法题，大全和菜鸟手册里面没有搜到，所以也不知道题号，不能在版内搜索，只好麻烦大家帮忙看看

Even though its per capita food supply hardly increased during two decades,stringent rationing and planned distribution have allowed the People's Repoblic of China to ensure nutritional level of 2000 calories  perperson per day for its population. 

a Even though its per capita food supply hardly increased during

b Even though its per capita food supply has hardly increased in

c Despite its per capita food supply hardly increasing over

d Despite there being hardly any increase in its per capita food supply during

e Although there is hardly any increase in per capita food supply for

c、d、e可以排除。a和b我拿不准。对于时态的问题，我一直搞不太清楚。网络课堂里面说在一句话中要前后时态一致（除了一些特殊情况），我的理解就是一个句子有“。”了，那么就是一句话，就要前后一致。但是之前作题又看到有在一个长句中，前后不一致的又不属于特例的。搞糊涂了~~

请指教！

我在新东方教材里看到，A的错误在于，during two decades  要用现在完成时。。。

但我又看到过说during the formation of the universe 用过去是。很困惑，同问。。

碰到during用什么时态。。