Which of the following is the best example of a response to external stimuli?
a. pupils of the eyes dilate in the dark.
b. a flu virus infection results in vomiting.
c. when a person is hungry, his/her stomach growls.
d. a fatigued person responds by sleeping.

Answer - A. Pupils of the eyes dilate in the dark.

Reasoning - To start reason why it is answer A is because of the changes in the environment. Since the changes in the environment is dark, then your eyes will begin to dilate or get bigger in the dark to capture more light. 

1. true.2. true.3. true.4. false.5. true.the concept of darwin's theory of evolution involves the idea that there are limited resources in a given living environment and that the organisms which live within have variations in their genetic make up due to random mating. the ones with beneficial genetic variations can express certain traits which give them an upper hand in surviving and reproducing in the said environment compared to ones who may have a more disadvantageous variation in traits. over time, the organisms with an upper hand will survive and reproduce more frequently and pass on their beneficial genes while the others die. the result is evolution, where we observe (usually, but not always) eventually, phenotypic distinct changes.