Up to now,our society __ with limited living space and a shortage of natural resources.
问题描述：
Up to now,our society __ with limited living space and a shortage of natural resources.
A.has been challenged B.is being challenged
为什么选A不选B
答
Up to now 暗含一个时间段,经常用来表示一个过程或者某种状态的持续.
意思是,目前为止,直到现在.
同义的有So far ,till now,它们都常用于现在完成时,且常用于句首.
Up to now he's still quiet.他直到现在仍保持沉默.