16.How are rocks grouped within each unit of geologic time similar? 17. A unit of geologic time is usually characterized by
of a dominant life-form.
18. What does the abbreviation Ma stand for?
19. When did Precambrian time begin?
20. What species were common during the Cambrian Period?

(select all the correct choose each statement that is scientific. - the opinions of randomly selected participants in a survey prove the idea that global temperatures are not increasing on earth. - the universe's average temperature and rate of expansion support the idea that it began as one super-dense and hot mass 13.8 billion years ago. - ocean tides are caused by the uneven gravitational pulls of the moon and sun on different parts of earth. - human life is more valuable than other forms of life on earth because humans are more intelligent than other organisms.