Each of four test tubes contains a different concentration of hcl(aq) at 25 degrees celsius. a 1-gram cube of zn is added to each test tube. in which test tube is the reaction occurring at the fastest rate

Given:

An experimental set-up where there are four test tubes containing different concentrations of hydrochloric acid, all at 25 degrees Celcius. All test tubes were added with 1 gram of Zn cube (at the same time). 

The reaction which occurs at the fastest rate is the test tube where HCl concentration is greatest. The high concentration of HCl enhances the speed of the reaction.