A puddle of dark liquid under a car indicates a leak in the oil or transmission fluid. Another warning sign of car trouble is uneven wear on tire treads. If the outer edge of the tire tread wears out before the center, the tire is under-inflated. The most significant early warning signal, however, is less and less mileage per gallon. Lower fuel economy is caused by a variety of problems ranging from a clogged air filter to ignition or carburetor trouble. The sentence that best expresses the implied main idea is:_______.

a. Several warning signs can indicate car problems.
b. A puddle of dark liquid under a car is a sign that a car has an oil or transmission-fluid leak.
c. The most significant warning signal of a car problem is getting lower mileage per gallon of gas.
d. Various problems may cause a car's fuel economy to decrease.