Which of the following is an indicator for sterilization by autoclave method

Bacillus stearothermophillus is used as the test organism. This is a thermophillus organism with an optimum growth temperature of 55-60degc and its spores require exposure of 12 min at 121degc to be killed Paper strips impregnated with (10)6 spores are dried at room temperature and placed in different They are then inoculated into suitable recovering medium and incubated for the sterility test at 55degc for five days