In 1979 lack of rain reduced India's rice production to about 41 million tons, nearly 25 percent less than those of the 1978 harvest.
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The sentence says that in 1979 shortfall of rain reduced India’s rice production to about 41 million tons. This amount was nearly 25% less than the production in 1978.
The important thing to understand here is that we are comparing the production in 1979 to the production in 1978. We are not comparing tons in the two years; ‘tons’ is just a unit of production.
Thus, the use of “those of” is incorrect.
(A) Incorrect. For the error mentioned above.
(B) Correct. The production of 1979 is compared to the production (or harvest) of 1978.
(C) Incorrect. The production is compared to a year, 1978.
(D) Incorrect. In addition to repeating the error in option C, this option also uses the incorrect comparison word “fewer” to compare productions of two years. “production” is an uncountable noun, so we cannot use “fewer”.
To learn more about countable and uncountable nouns and the appropriate comparison words for them, please read this article.
(E) Incorrect. The use of “fewer” is incorrect, as explained in option D. Also, “that of” is incorrect, as explained in the sentence analysis.