题目内容：
Questionsare based on the following passage:(79) It' s a day to celebrate, a time to educate, and a wa~/to make a difference. Earth Day was first observed on April 22.1970. The tradition began in the United States and is now an in-ternational focal point for people who are concerned about the environment.
Imagine a time before the Clean Air Act, the Clean Water Act, the Endangered Species Act,and even the Environmental Protection Agency. Hot topics of the time were industrial pollution,nuclear accidents, oil dependency, pesticides, and a variety of social issues. However, policy-makers were not yet acting on many important environmental problems,Earth Day grew out of grassroots action and institutional support. Student leader Denis Hayes and Wisconsin Senator Gaylord Nelson were two key organizers for the first event. Over 20 mil-lion concerned citizens rallied for clean air and water. College students were a major force in the campaign, although participants of all ages were involved. Still, we cannot really credit a handful
of individuals for the success of Earth Day. After all, a major message of Earth Day is that every single person has an important role in the environment.
(80) The first Earth Day was very successful, inspiring the passage of congressional protec-tion for air, water, and endangered species. Momentum from the demonstration also fueled theformation of the Environmental Protection Agency.
Thirty years later, the focus has changed but the urgency remains the same. Due to increas-ing amounts of scientific evidence that fossil fuels are changing the global climate, the theme for Earth Day 2000 is Clean Energy. The four major goals of Earth Day 2000 are clean cars, clean power, clean air, and clean investments.
April 22nd, 2000 is the __________years' anniversary of the Earth Day. A．30th
B．20th
C．10th
D．100th
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