Workers need to take which of the following common-sense steps to protect themselves from falls through skylights and roof and floor openings?

a. never sit on, lean against, or step on a skylight lens or any covering placed over a hole in a roof or floor
b. guard or securely cover all holes created or uncovered before leaving a work area
c. always use a pfas when working over an unguarded or uncovered opening more than six feet above a lower level
d. all of the above