4) Complete the following statement: Different elements... A) Must have a mass number of at least 18.
B) Have more electrons than protons.
C) Have the same number of protons.
D) Have different numbers or protons.

Which of the following has expanded our knowledge of the universe beyond our solar system the most? a. manned space travel b. the hubble space telescope c. the pioneer and voyager missions d. the international space station