答案应选 2.one；
one 后省略了 invention,直接把下面两条句子组合成了复合句：
The first great invention was the wheel.
One invention that is still important (today) is the wheel.
组合后的句子也可以换种方式排列而意思不变：
The first great invention was the wheel,one that is still important.
这样就看得比较清楚成分：the wheel 是宾语,而 one 引导从句作为宾补.