2,3−Dimethyl−2−butene can be prepared by heating which of the following compounds with a strong acid ?

a. (CH₃)₂C = CH - CH₂ - CH₃
b. (CH₃)₂CH - CH₂ - CH = CH₂
c. (c)(CH)CH - CH - CH = CH₂
CH₃
d. (CH₃)₃C - CH = CH₂