Which of the following is the correct order for the reactivity of various substrates in SN1 reactions (ranked from most reactive to least reactive) ?
a) 3° > 2° > 1° > methyl
b) Methyl > 1° > 2° > 3°
c) 1° > 2° > 3° > methyl
d) 3° > 1° > 2° > methyl