听力原文： For the past few weeks we have been discussing national energy conservation alternatives for the future. Today I am going to talk about what one community is presently doing to conserve energy. The people of Davis California have succeeded in cutting their energy consumption by one third since 1973. The first energy saving action that was taken in the early 70' s was the legislation of strict building codes; all new houses in Davis must have the proper insulation so that heat will not escape unnecessarily in the winter. New houses also face north or south so that they will not be overheated by the sun in summer. The laws have had a definite effect. Since 1976, there has been a 50% saving in the amount of natural gas and electricity used in heating and air conditioning. There are other energy saving features about Davis. Buses, partially supported by the city, transplant university students throughout the area. There are twenty-four miles of bicycle paths and today there are twice as many bicycles as cars in the city. By reducing the available parking space the city council has succeeded in reducing the number of cars in the city every day. Another benefit of the reduced parking space is the greater number of small cars. People are saving gas because they are choosing not to drive or because they are driving fuel economic cars. Davis California has become an energy saving model for other cities. Time is up for today. Next week we will return to our regular topic of national energy alternatives.
What is the main topic of the lecture？

A. Bicycles and cars.
Building codes.
C. Energy conservation.
D. New housing construction.