Why are transport proteins multipass rather than single-pass integral membrane proteins?

Transport proteins multi-pass so that multiple alpha helices can form a hydrophilic pore that is able to diffuse small molecules. If transport proteins were single-passing, the alpha helix would not have a core with a diameter large enough to diffuse solutes.
Multi-pass membrane proteins loop in and out, while single-pass membrane proteins cross the membrane once.

The correct answer would be b