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Light is modeled as traveling in straight lines called rays. Light may change direction when it encounters objects (such as a mirror) or in passing from one material to another (such as in passing from air to glass), but it then continues in a straight line or as a ray.

Reflection occurs when light traveling through one material bounces off a different material.

Refraction occurs when  a light wave, incident at an angle away from the normal, passes a boundary from one medium into another in which there is a change in velocity of the light.

Explanation:

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d is the best answer for this question.

1. ford: gerald ford´s presidency, from 1974 to 1977, faced high inflation, named it public enemy nº1, and created the win program (whip inflation now).

2. carter: president from 1977 to 1981, carter had controversial poitions over abortion, but yet, he enforce the roe vs wade decision, which had pro-choice content.

3. reagan. for his eloquency and powerful speeches in his mandates, from 1981 to 1989, reagan earned this nickname, “the great communicator” .

4. george h w bush: despite many presidents had important roles in managing and trying to end the cold war, it was in bush sr's mandate that the berlin wall was materially destroyed, and he was visited by gorbachev in the uspresident from 1989 to 1993.

5. clinton: defense of marriage act. clinton signed this controvesial act, which defined marriage and spouses in strict heterosexual perspective, that was later declard unconstitutional by the us supreme court.

6. george w bush: in the begining of his presidency, which lasted until 2009, bush jr faced the greatest terrorist attacks in us and probably the worlds history, in september, 11 of 2001.

7. obama: in attempts to cease a great economical crisis, barack obama signed the controversial recovery act in february 2009, which handled 780-billion dollars.