1.Mistake in three of the following will make a contract voidable. Which one will not?

a.Mistake as to the substance of the thing which is the object of the contract.

b.Mistake as to the principal conditions which principally moved one or both parties to enter into the contract.

c.Mistake as to the identity or qualifications of one of the parties, which identity or qualifications have been the principal cause of the contract.

d.Simple mistake of account.