Following the dolphins in the murky water is extremely challenging. 在浑浊的水中跟随印度河豚十分困难
but life ahead will be challenging, 但前方的生存之路仍充满荆棘
Mastering precipitous terrain is always challenging, 在峭壁上行走固然很难
but finding enough food to survive is so challenging that most animals living here tend to be small 但由于寻找食物是个巨大的挑战 因此很多雨林生物都长得很小巧
This is a challenging time for the bear family. 对于北极熊一家来说 这是一个极富挑战性的时刻
But each year over 50,000 demoiselle cranes set out on one of the most challenging migrations on Earth. 可是 5万多只蓑羽鹤每年都会 进行一场地球上最具挑战性的迁徙