(c) In the case of a block and tackle system, the mechanical advantage increases with the increase in the number of pulleys.
(d) The lower block of a block and tackle pulley system must be of negligible weight.

Solutions:

(c) Because the mechanical advantage is equal to the total number of pulleys in both blocks, the mechanical advantage grows as the number of pulleys increases.
(d) Because the mass of the lower block affects efficiency, the weight of the lower block of pulleys reduces efficiency.