hypothesis testing

So, this is a long problem and I am sturggling where to start. If someone wants to just help me get the variables down and the hypothesis I can try to figure out the answer

After several years of studying human performance in flight simulators, a psychologist knows that reaction times to an overhead emergency indicator form a normal distribution with mean = 200 milliseconds and standard dev = 20. The psychologist would like to determine if placing the indicator in front of the person at eye level has any effect on reaction time. A random sample of n = 25 people is selecte, they are tested in a simulator with the indicatior light at eye level and their reaction times are recorded. Use a .05 alpha.
a) identify independent and dependent variable
b) State the null hypothesis using a sentence that includes the dep. and indep. variables

- I am thinking that the indep variable is the sample of 25 and the dependent variable is the standard dev of 20??