You start with 400 atoms of a radioactive isotope that has a half-life of 10 years. draw a plot indicating how many atoms of the isotope are left after 1, 2, and 3 half-lives. time should be on the x-axis and number of atoms on the y-axis. be sure to label the axes and indicate the number of atoms left after each half life.

A,b,c is the answer to the question

Answerbecause of improved transportation, communication, and other advantages;

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1. why do astronomers think that there may be other advanced civilizations?
because  it was discovered that there are other planets like earth out there, able to  accommodate life forms.
2. what technologies are astronomers using to look for life on other planets? why?  
they  have used radio telescopes, but now they use laser telescopes   to track laser communications.