Obama also called climate change and nuclear proliferation "challenges that neither of our nations can solve by acting alone". He said the two will continue to "build a positive, cooperative and comprehensive relationship".

【正确答案】奥巴马也把气候变化和核扩散称为“两国都无法单方面应对的挑战”。

【答案解析】 原句中“call sth...”译为“把……称为……”。此外，that引导的从句修饰challenges，英语中定语从句后置，但是汉语的定语却习惯前置，放在被修饰的名词前面。因此，应注意适当调整语序。