目前，非洲社会经济虽然发展很快，但可持续发展还面临许多问题，主要有（  ）

①人口增长过快②粮食短缺③矿产资源短缺④水土流失、土地荒漠化．

下列等式一定成立的是（    ）

选词填空。

    What was I like when I was young? My mom said l was lovely．I was____in everything．I liked oil____．I liked riding a horse．I liked____a cow．I liked helping____vegetables on the farm．I also liked shopping．At the supermarket I didn't buy____cheap．I was only interested in expensive things．Haha，I was a funny boy when I was young，wasn't I ?

下列的哪一项不是动物在自然界中所起的作用？（　　）