The hyperconjugative stabilities of tert-butyl cation and `2`-butene, respectively, are due to
A. `sigma rarr p` (empty) and `sigma rarr pi^(**)` electron delocalisations
B. `sigma rarr sigma^(**)` and `sigma rarr pi` electron delocalisations
C. `sigma rarr p` (filled) and `sigma rarr pi` electron delocalisations
D. `p`(filled) `rarr sigma^(**)` and `sigma rarr pi^(**)` electrons delocalisations