Please discuss the following three questions:

$1.$ Explain tthe meaning of $($a$)$ differential revenue, $($b$)$ differential cost, and $($c$)$ differential profit $($loss$).$

$2.$ A company accepts an incremental business as a special price that exceeds the variable cost. What other issues must the company consider in deciding whether to accept the business?

$3.$ In the long run, the normal selling price must be set high enough to over what factors?