2个语法问题

1。the question is whether tourists will continue to visit game parks and see rhinoceroses after their horns are trimmed

答案是whether tourists will continue to visit game parks and see rhinoeroses onece the animals' horns have been

请问这里after和once有什么很大区别吗？为什么要选once这个选项？谢谢

2。His studies led him in1837 to propose the concept of an age in which great ice sheets existed in what are now temperate areas.

原文是正确答案，但请问ice sheets exist应该在led in 1837之前，为什么不用had existed呢？还有就是这里的what怎么理解？当成the area that么？谢谢！

1. 关键不在after和once,而在their的指代上

2. what从句可以看成是一个名词短语但又充当句子成分。如A logarithm is what is known in algebra as an exponent.对于完成时态，我不知道语法上该怎么说，我的感觉就是ice sheet existed只是一个concept，类似一个公理，只是跟"now"对比，exist过，如果用完成时态，怪怪的。