Read this excerpt from a science textbook and answer the question.
the sun is the star at the center of our solar system. it is truly massive. our sun is so large that it contains over 90% of the matter in the entire solar system. that means that nine out of ten atoms belong to the sun!
our star is able to produce heat and light because of a process known as fusion. the sun does not have a crust like solid planets do. instead, the visible edge of the sun is the edge of its photosphere. inside the sun, temperatures can reach over 13,000,000 kelvin!
the words "matter," "photosphere," "fusion," and "kelvin" are bolded in the paragraph above because
a. bold words are defined in the book's glossary.
b. bold words are always from foreign languages.
c. bonus words are always bold in a textbook.
d. words that are hard to understand are bold