There are more than 100 million cars in the United States. An ordinary car gets less than 15 miles from each gallon   of gas. It travels about 10,000 miles each year. In that time, it uses about 650 gallons of gas. In all , automobiles use up some 70 billion gallons of gas a year. That comes out to be four-and-a-half million barrels a day. The importance of saving gas, then, cannot be stressed too much. Let's say, the fuel used by each car could be cut back just 15 percent. This could be done by making fewer trips each day. It could be done by keeping the automobiles in good shape. It could be done through better driving habits. If these were done, the nation's use of fuel would fall by close to two-thirds of a million barrels per day. Everyone can help to save gas. One way is to ride buses. Some could walk to work. Others could ride their bikes. Another way is to share a ride. People could share cars. About one-third of all cars are used for going to and from work. You can go shopping with a friend from time to time. If two people use a car instead of two, they all save. Driving stress   would be less, too, with fewer cars on the road. The savings on gas around the nation would come to more than one-half million barrels a day. Still another way to save is by cutting out trips one does not need to take. Can you find one car trip per week that could be done by telephone? Can you combine trips? If each car travelled 10 miles less each week, they could save three-and-a-half billion gallons of gas a year. This comes to nearly 5 percent of the total passenger cars demand for gas. The way people drive decides how much fuel they save. Careful drivers may get 20 percent more miles per gallon than ordinary drivers. They obey the 55-mile-per-hour speed limit. If just one gallon of gas were saved each week for each car in the country, a total of five-and-a-half billion gallons could be saved a year. According to the passage, it is possible to save five percent of the total passenger cars demand for gas each year, if  _  .
people all share car-riding instead of owning their own cars all the cars cut off one trip of 10 miles each week two people go shopping in a car from time to time people use telephones instead of riding cars