As he moved farther into the cave, he noticed that the water was moving ________and not towards the open air.
句意： 当他往洞穴深处移动时他发现水是在向内而不是向外面流动。 由于后半句 not towards the open air可知水应该向相反方向流动， inwards 意为“向内地， 向中心地”， 符合语境。