In the given question, a part of a sentence is printed in bold. Below each sentence, some phrases are given which can substitute the bold part of the sentence. Find out the phrase which can correctly substitute that part of the sentence. If the sentence is correct as it is, mark the answer as 'No correction required' or 'No Improvement'.
Not long back, in Japan, a deadly nerve gas affected a large number of people.
Infected
Not long back, in Japan, a deadly nerve gas infected a large number of people.