Which of the following torque fluctuations represent the most critical situation according to Soderberg criterion?

Question

The yielding strength of a steel shaft is twice its endurance limit. Which of the following torque fluctuations represent the most critical situation according to Soderberg criterion?

      (a) –T to +T                                                        (b) –T/2 to +T

      (c) 0 to +T                                                           (d) +T/2 to +T