A longer exposure time is needed for a dry heat oven because:

1) Dry heat has limited killing action.
2) Dry heat ovens can only reach a maximum temperature of 200 degrees F.
3) Dry heat penetrates slowly and unevenly.
4) Microorganisms and spores are more resistant to dry heat.
5) All of the above.
6) c and d.

Longer exposure time in a dry heat oven is required due to the slow and uneven penetration of dry heat and the higher resistance of microorganisms and spores. The correct answer is option 3) dry heat penetrates slowly and unevenly.

Explanation:

A longer exposure time is needed for a dry heat oven because dry heat penetrates slowly and unevenly and microorganisms and spores are more resistant to dry heat. Moist-heat sterilization is more effective at penetrating cells and typically requires shorter times for sterilization, while dry heat sterilization requires a longer duration at elevated temperatures (up to 170 °C) due to the nature of its killing action. In dry heat sterilization, different microorganisms will respond differently to high temperatures, with some being more heat tolerant, necessitating longer exposure to ensure all microbes are adequately killed. Therefore, answer 6) 'c and d' correctly addresses the student's question.

Dry heat ovens require a longer exposure time because dry heat penetrates slowly and unevenly. Unlike moist heat, which is more effective at killing microorganisms because it penetrates cells better, dry heat has limited killing action. Also, microorganisms and spores are generally more resistant to dry heat. Therefore, the correct answer is option 3) dry heat penetrates slowly and unevenly.