One of the ACT’s preferred tricks is to give you a sentence that looks like the following:

Millions of pedal-assisted electric bicycles have already been sold in Asian countries, such as China, where bicycles often serve as a cheap, yet efficient mode of transport.

A. NO CHANGE
B. cheap; yet efficient
C. cheap, yet efficient
D. cheap yet efficient

Because it seems natural to pause before the word yet many people’s instinct is to insert a comma at that point in the sentence.

However, the rule is that when two adjectives are separated by a conjunction (typically but or yet, although and could be tested as well), no comma should be used before the conjunction.

In this case, cheap and efficient are both adjectives, so no comma is necessary. The answer is therefore D.

You can also think of the rule this way: comma + but/yet = period.

When you plug in a period, you get nonsense:

Millions of pedal-assisted electric bicycles have already been sold in Asian countries, such as China, where bicycles often serve as a cheap. Efficient mode of transport.

If a period doesn’t work, neither does comma + but/yet.