44. __________ The most frequent cause is an air mixture that is too rich; that is, the fuel-air ratio is greater than 10:1. The second most frequent cause is oil in the fuel in concentrations greater than 60 cc per gallon of fuel. The result of such heavy concentrations of oil is seen in the dense blue exhaust smoke. A third, and somewhat less frequent, cause is the speed of the engine itself. The lower the speed, the greater the density of smoke from oil burning in the combustion chambers.
(A) Research into the causes of excessive smoke in the exhaust systems of small internal combustion engines shows that there are several factors responsible.
(B) Research into excessive smoke in the exhaust systems of small internal combustion engines shows no significant correlation between amounts of smoke and the following three pollutants.
(C) Research into excessive smoke in the exhaust systems of small internal combustion engines shows three main causes: a too thin air mixture, a low concentration of oil in the fuel, and fast engine speed.
(D) Based on research, excessive smoke results in three major kinds of pollution: a too thin air mixture, a low concentration of oil in the fuel, and fast engine speed.