A mining company will spend $28 million in order to exploit a low-grade placer depoist of gold ore. They estimate the depoosit will produce profits of $6 million per year for 6 years. They calculate the NPV using an estimated cost of capital of 6%. What is the maximum that the cost of capital can deviate from this estimate that still makes the decision to mine worthwhile?

a) 1.66% b) 1.69% c) 1.72% d) 1.60%