In the following sentence, a part of the sentence is underlined. Below are given alternatives to the underlined part, which may improve the sentence. Choose the correct alternative. In case no improvement is needed, choose the option that indicates 'No improvement'.

Cryptozoology is not a recognized branch of zoology but a pseudoscience because it relied heavily upon anecdotal evidence, stories and alleged sightings.

A. It has been relying heavily upon

B. It relies heavily upon

C. It relied heavily on

D. It rely heavily upon