Decreasing rate of reaction between I2(s) and H2(g)?


> 
> Which change will decrease the rate of the reaction between
>  $\ce{I2(s)}$ and $\ce{H2(g)}$?
> 
> 
> (A) Increasing the partial pressure of $\ce{H2(g)}$
> 
> 
> (B) Adding the $\ce{I2(s)}$ as one piece rather than as several small ones 
> 
> 
> (C) Heating the reaction mixture 
> 
> 
> (D) Adding a catalyst for the reaction
> 
> 
> 


I can eliminate D as a catalyst would speed up the reaction. 


I am not sure about the others though. I could look up the enthalpy for this reaction and eliminate C, but this is from the Chemistry Olympiad and I can't look up enthalpies during the test. Is there any other way to determine that this reaction is endothermic?


Once you eliminate C, you are left with A and B. How would you go about finding the right answer?


