Standard Costing

This is a BPP MCQ.
Which on the following is the least likely reason why standard costs might not easily applied to road haulage and distribution services.

A) It is difficult to measure labour times reliably
B) Variable costs are gegligible
C) It is difficult to identify a standard item for costing
D) Standard costing applies to manufacturing industries only

The Correct answer is C. BPP’s explanation is as follows
(With road haulage and distribution, drivers’ times on the road are measure automatically. Variable costs can be high (labour and fuel, for example). Standard costing is more common in manufacturing but in principle can be applied to service industries. The problem is to identify a standard item for which a cost can be measure and variances subsequently calculated. In road haulage for example a standar measure may be cost tonne/kilometre delivered : this does not lend itself easily to variance analysis.

I think the question should be the most likely reason not the least likely reason.