First of all, this is the most disgusting question I've ever seen. I'll definitely pick "than normal" rather than "is normal" in the first place because it sounds so right. But after the second thought, I believe Princeton is right. Second, you're right, "than usual" is a correct expression confirmed by OG. 253. Analysts blamed May's sluggish retail sales on unexciting merchandise as well as the weather, colder and wetter than was usual in some regions, which slowed sales of barbecue grills and lawn furniture.
(A) colder and wetter than was usual in some regions, which slowed
(B) which was colder and wetter than usual in some regions, slowing
(C) since it was colder and wetter than usually in some regions, which slowed
(D) being colder and wetter than usually in some regions, slowing (B)
(E) having been colder and wetter than was usual in some regions and slowed
ETS explains was before ususal in OG253 is unnecessary. How come in Princeton Review, is before normal is ncessary? The line of reasoning is stated as below: 1. "Ususal" is an adj., and adj. alone. Therefore, the phrase that the weather was colder and wetter than usual could be interpreted without confusion. That is, the weather was colder and wetter than (the) usual (weather is). Than usual is concise and idiomatic to state the idea. 2. "Normal", on the other hand, could be an adj. or a noun. As a result, than normal causes confusion. If "normal" is considered an adj., it means The temperature is greater than (the) normal (temperature is). If "normal" is considered a noun, it means The temperature is greater than normal(n.). This comparison is different from the above. That is, "normal(n.)" is referring to an event---the temperature is greater, and it is greater than normal--- rather than an object, the temperature. In conclusion, again, this is a disgusting question. | 
