$20.$ A client's insurance policy includes this provision: "This policy contains all the agreements between you and us concerning the insurance afforded.... This policy's terms can be amended or waived only by endorsement by us and made a part of the policy." According to this provision in the contract between the insurance company and the insured,

a$.$ a written certificate of insurance is considered an endorsement.

b$.$ a written certificate of insurance issued by the insurance agent who represents the insurer can be used to modify the policy.

c$.$ the terms of this policy apply unless the insurance agent who sold the policy issues a written endorsement that becomes part of the policy.

d$.$ the terms of this policy apply unless the insurance company that issued the policy issues a written endorsement that becomes part of the policy.