One Reason That the Great Reformer Mikhail Gorbachev Rose to Supreme

One reason that the great reformer Mikhail Gorbachev rose to supreme power in the Soviet Union was that:

A) his advanced age and experience made him greatly respected.
B) a younger man was desirable because his immediate predecessors had been too old or became incapacitated in office.
C) his leadership of the KGB gave him immense power.
D) the Soviet system tended to push able reformers to the top every few years.