单项选择题
The technology exist today to operate automobiles on CNG and LPG which would meet the second objective. In fact, buses and trucks in many urban areas of Asia and Europe operate on CNG and LPG. Expansion of the use of clean-burning gaseous fuels (CNG, LPG and hydrogen) for private automobiles, however, has been slow. Among the obstacles to overcome are low-energy storage density (total miles driven per filling of fuel tank), poor economics (the low price of petroleum-based products have historically not provided much incentive for turning to these alternatives) and major engineering challenges in adapting the current internal combustion engine to these fuels.
A、In fact, buses and trucks in many urban areas of Asia and Europe operate on CNG and LPG.
B、It was slow to expand the use of clean-burning gaseous fuels.
C、There are several obstacles to the expansion of the use of clean-burning fuels.
D、CNG and LPG would meet the second objective.
