Cave exploration,or pot holing,as it has come to be known,is a relatively new sport.Perhaps it is the desire for solitude or the chance that they _______ an unexpected discovery that lures people down to the depths of the earth.

a) would make   b) can make   c) may make  d) might  have made

以上哪一個是最佳選項？具體講一講b項與c項的區(qū)別。謝謝！