Mark agrees to buy Dale's one-half-ton truck so that he can pull his trailer. Both Mark and Dale believe that the truck is large enough to do the job. After they complete the deal, Mark hitches the truck to the trailer and attempts to pull the trailer, but without success. The contract between Mark and Dale can be rescinded because of:

a. an allowable mistake.
b. a mutual mistake.
c. a beveled mistake.
d. a unilateral mistake.