选择___the enormous flow of food from the entire globe,these c

选择___the enormous flow of food from the entire globe,these countries have for many years not
___the enormous flow of food from the entire globe,these countries have for many years not felt any population pressure
a,thanks to b,by means of c,in line with d,with regard to
为什么选a? b是用 ,依靠的意思吗 为什么不选b? c d分别是什么意思