At said hearing, or adjournments thereof, the board shall review said roll and determine the continuous base benefits to land and buildings furnished continuous protection by the improvement system of the district; hear objections to the adoption of said roll; correct, revise, change, modify or set aside such roll, or any part thereof, as to the board shall appear equitable and just; and then adopt the same by resolution. All objections to this or any subsequent roll must be in writing and filed with the board during the hearing before the roll is adopted and must state clearly the grounds of objection. Objections not made within the time and in the manner herein prescribed shall be conclusively presumed to have been waived.