Answer: a. Often external physical work cannot be easily measured.
Step-by-step explanation:
Physical work or manual labor is done by people as a part of their daily routine. This involves the movement of muscles and bones to a limited to considerable extent. It involves the activities like cooking, cleaning and moping, riding cycle and other activities which requires labor.
These physical works cannot be considered as a part of exercise as these are not considerably measured. But they can be included as a part of exercise as in both cases manual labor is required.