Which structures represent 2-methylbutane (c5h12)? check all that apply?

Structures that represent 2-methylbutan: a, b, c, d.
a - because methyl group is closer to the right end of butane chain, numbers go from right to left in butane and methyl is on second carbon.
b - because methyl group is closer to the left end of butane chain, numbers go from left to right in butane and methyl is on second carbon.
c - longest chain is butane, from one CH₃ to another CH₃.
d - same as a).