If the degree measures of the angles of a convex quadrilateral are in the ratio~$3:4:5:6$, by how many degrees does the measure of the largest angle exceed the measure of the smallest angle? 
选项为:A:$30^{\circ}$  B:$45^{\circ}$  C:$60^{\circ}$  D:$75^{\circ}$  E:$90^{\circ}$  请回答其中一个选项.