Someone please help me

Astudent reacts 13 moles of iron with 21 moles of oxygen according to the following equation:

1) which of the following is the best example of a physical change? a) sugar dissolving in tea b) firefly glowing 2) in the combustion of ethane, what is/are the reactants? c2h6 + o2 ==> co2 + h2o a) c2h6 and o2 b) co2 and c2h6

In a ball-and-stick molecular model, what do the sticks represent?