Which of the following represents the basic distinction between a bilateral contract and a unilateral contract?

a$.$ Only one promise is involved if the contract is unilateral, but two are involved if it is bilateral

b$.$ The statute of frauds applies to a bilateral contract but not to a unilateral contract.

c$.$ Rights under a bilateral contract are assignable, whereas rights under a unilateral contract are not assignable

d$.$ Specific performance is available if the contract is unilateral but not if it is bilateral.

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