Which of the following statements is correct concerning letters for underwriters, commonly referred to as comfort letters?

a. Letters for underwriters are required by the Securities Act of 1933 for the initial public sale of registered securities.

b. Letters for underwriters typically give negative assurance on unaudited interim financial information.

c. Letters for underwriters usually are included in the registration statement accompanying a prospectus.

d. Letters for underwriters ordinarily update auditors' opinions on the prior year's financial statements.