1."I will sell you this bike for $10,000 cash".

a.This is a valid offer, but only if it is expressed in writing

b.This is not a valid offer because it only invites the other party to negotiate

c.This is a valid offer because it expresses a willingness to conclude the deal if the other party agrees to pay the required $10,000

d.This is not a valid offer because it doesn't express the terms with sufficient clarity and definiteness