Nitrous oxide, commonly called laughing gas, has been a matter of concern to environmentalists recently because

Question

$\begin{array}{1 1} \text{a. it is thought to cause cancer at low concentrations. } \\ \text{b. it produces photochemical smog. } \\ \text{ c. it is a greenhouse gas.} \\ \text{ d. none of the above} \end{array} $