According to the pauli exclusion principle, when can two electrons occupy the same orbital?

a) only if there is no place else to put them
b) only if they have the same spin
c) only if they opposite spins
d) only if they're alike in all of their properties

Indicate whether the specified alkyl halides will form primarily substitution products, only elimination products, both substitution and elimination products, or no products when they react with sodium methoxide. 1-bromobutane 1-bromo-2-methylpropane 2-bromobutane 2-bromo-2-methylpropane