First off, I want to stress that this question is actually not MUst-be-true one. It is actually a 'Straighten' question type.
As for the question, the correct answer is B
Premise-Despite the approach of winter， oil prices to industrial customers are exceptionally low this year and likely to remain so.
Conclusion -Therefore， unless the winter is especially severe， the price of natural gas to industrial customers is also likely to remain low.
Even if u cant understand which one is the conclusion by simply reading the argument, the word "therefore" is a great indicator of having a conclusion.
So, if u see that a sentence begins with the word "therefore", be sure that this sentence is a conclusion.
As we can see the premise says about the oil prices, but the conclusion is made about the gas prices. So, we have some logical gap between these two. In order to link these two sentence, we need an answer choice, that will have this link and by doing so, will definitely strengthen the passage above.
answer choice B states that the industrial users who consume most natural gas can quickly and cheaply switch to using oil instead.
the answer choice links gas with oil. it means that if the oil prices are low, the gas prices will also be low.
Happy are those who dream dreams and are ready to pay the price to make them come true