题目选项分析

As sources of electrical power, windmills now account for only about 2,500 megawatts nationwide, but production is expected almost to double by the end of the year and thus to provide enough electricity for 1.3 million households.

As sources of electrical power, windmills now account for only about 2,500 megawatts nationwide, but production is expected almost to double by the end of the year, which would thus be providing enough electricity for 1.3 million households.