Comprehension: In the following passage, some words have been deleted. Read the passage carefully and select the most appropriate option to fill in each blank.
 Nitrogen is essential to life (1)______ Earth. It is a component of all proteins, and it can be (2)______ in all living systems. Nitrogen compounds are (3)______ in organic materials, foods, fertilisers, explosives and poisons. Nitrogen is crucial to life, (4)______ in excess it can also be (5)______ to the environment.

Q:

Select the most appropriate option to fill in blank number 1. 

• 1
  at
• 2
  into
• 3
  on
• 4
  In