Express the sum of 9.78 g and 7.0 g using the correct number of significant
digits.

C. 16. 8 g

Explanation:  

when we are using significant numbers we have to keep in mind if the last digit is over 5 or down 5 to increase or decrease 1 digit the number to the left, if the last number is down 5, we decrease the number to the left 1 digit, but if the number is over 5, we can increase 1 digit to the number to left.

then we have this equation, as the 0 is not a significant number, we can omit this one.

9.78 g + 7.0 g = 16.78 g

as the last number is over 5 we can increase 1 digit to the number to the left, the we get the following result.

16.78 g = 16.8 g

because of that the correct answer is C. 16. 8 g

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.

the answer is b because, investigating whether another species of moth also drinks tears might be interesting, but it would not directly explain why this species of moth does.

to explain why this species of moth drinks tears, the scientist could ask: "what nutrients that moths need are in elephant tears? "

the scientist could experiment with moth repellent or oter ways to keep the moths away, but that would not answer his original question.