The following sentence issues which principal part of the verb? i ring the bell when it is time for the doors to open .
a past participle
b past
c present participle
d present

Ring- present participle

Explanation:

The sentence is an example of "present indefinite sentence" in the affirmative version which suggests the use of the first form of the verb i.e. "present participle" form of the verb. The statement exemplifies a habitual action of the speaker which is written with the use of present participle form of the verb. "I ring the bell when it is time for the doors to open". "ring"(present participle) here is the verb that draws the answer of the question.

the answer is industry

pandas are endangered because of habitat destruction by humans.

as of 2014 there were around 2,000 left, since then, it supposedly has risen 17%.

they are a very endangered species.

-theoneandonly003