All trees belonging to species A have started to die mysteriously if left untreated. Affected trees ...

All trees belonging to species A have started to die mysteriously if left untreated. Affected trees that are treated with a fungicide recover. Also, trees in parks that have been treated with broad-spectrum insecticides will not get sick.
 
  Which of the following is the most parsimonious explanation for the dying of trees belonging to species A?
  a. The trees are killed by the feeding activity of insects alone.
  b. The disease is vectored by insects.
  c. All trees are killed by a wind-borne fungal disease.
  d. Some trees are killed by fungal infections while others are killed by insect feeding.
  e. A and B are equally parsimonious.