Darius thinks that if the motors in motorboats are redesigned to not leak oil, they can be made environmentally friendly. What is the main problem with Darius's theory?
a) Motorboats have other environmental impacts besides oil leaks.
b) Redesigning motors to not leak oil is technologically impossible.
c) Oil leaks are not significant contributors to environmental damage.
d) Motorboats are already environmentally friendly without modifications.