Ecosystems and populations change

Use the following information to answer the next two questions.

In this environment, the spraying of the insecticide appears to act as

1.   
a) an agent to eliminate old and weak insects
   
b)a biological barrier
   
c) a growth inhibitor
   
d) an agent of selective pressure


I put d as the answer but im not sure if its correct please help.

2.The resistance to the pesticide that occurred in the insects was developed by:
 
a) a favourable change in the DNA of the insects
   
b) the food source of the insects
   
c) chance and will not occur again
   
d) the insecticide DDT

I think the answer is a but again im not sure, so please help.

The given information for both of theses questions is on the thumbnail attachment, so click on it to view the information. thx.