Question

Please explain why the answer to question 3.1 is C.

Answer 1

The author introduced a clause within the sentence explaining how ‘air conditioning has taken root among big corporations’ was at first introduced by a DASH, but then later concluded with a comma. In order to make this consistent, one must do either of the following:
1. ...air conditioning—which is blah blah blah....powerful corporations—uses the simple power of ice. (replace comma with dash at the end)
OR
2. ...air conditioning, which is blah blah blah....powerful corporations, uses the simple power of ice. (replace dash with comma at the beginning)

Basically, when introducing a clause when breaking a sentence, either use 2 commas or a dash. dont use a mix of both. Hope this helped!