Which statement BEST describes a firm commitment undwrwriting $?$ A The underwriter agrees to buy all of the issue and assumes the risk for any unsold securities$.$ B The underwriter agrees to use its best efforts when selling the securities offered in the issuance. C The underwriter agrees to use its reasonable efforts when selling the securities offered in the issuance. D The underwriter agrees to sell the entire allotment of the offering within $30$ days or the offering is terminated.