15. 2,3-Dimethyl-2-butene can be prepared by heating which of the following compounds with a strong acid ?
a)
b) \( \left( CH _{3}\right)_{3} C - CH = CH _{2} \)
c) \( \left( CH _{3}\right)_{2} C = CH - CH _{2}- CH _{3} \)
d) \( \left( CH _{3}\right)_{2} CH - CH _{2}- CH = CH _{2} \)
