Richard, an engineer, supervises the construction of a new mountainside roadway. When the road collapses in a landslide due to faulty grading, Richard is sued by motorists and hikers injured in the collapse. As a professional engineer, Richard is held to the same standard of care as

a$.$

those injured in the collapse of the bridge.

b$.$

other engineers.

c$.$

other professionals, including doctors, dentists, and lawyers.

d$.$

ordinary persons., a$,$b$,$c or d