A circle is centered at $O$, $AB$ is a diameter and $C$ is a point on the circle with $\angle COB=50^{\circ}$. What is the degree measure of $\angle CAB$? 
选项为:A:$$20$$  B:$$25$$  C:$$45$$  D:$$50$$  E:$$65$$  请回答其中一个选项.