A student observes some firewood as it is heated and burns. The student concludes that a chemical reaction has occurred. Which of the following observations about the burning wood provides the best evidence of a chemical reaction?

i do not get it   at all

substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.