In the following passage, some words have been deleted. Read the passage carefully and select the most appropriate option to fill in each blank.

The environment of the earth is a great (1)___ to mankind. It is a pity that this environment is in grave danger of being (2)____, and that this danger is being posed by man himself. Humans have seldom given thought to the fact that the air they breathe, the water they drink, or the food they eat could be (3)____. They have always assumed the bounties of nature to be inexhaustible. There is little (4)_____ on their part that what they consider progress is seriously disturbing the ecological balance and (5)_____ to the breakdown of the lifesupporting systems on the earth.