Question 27:
The correct answer is B. By making small changes in our energy use, we can make a big difference for the environment.
This option uses the correct verb form "making" and the correct preposition "in."
Question 28:
The correct answer is A. Last year, we visited Son Doong Cave, which is one of the largest natural caves in the world.
This option uses the correct superlative form "largest" and the correct preposition "in."
Question 29:
The correct answer is D. Be careful because building work is happening.
The sign clearly indicates that construction is in progress, so caution is advised.
Question 30:
The correct answer is B. You need a special type of ticket if you travel regularly.
The notice specifically mentions the "Super Weekly Saver Ticket" for regular travelers, implying that a special ticket is required.