Select the correct statement:
(a) A wheel barrow is a lever of class I
(b) The efficiency of a machine is always $$100\%$$.
(c) Friction in moving parts of a machine reduces its efficiency.
(d) No lever has the mechanical advantage greater than $$1$$.
(e) It is easier to lift a load vertically up than to push it along an inclined plane.
(f) A screw is made two inclined planes placed together.