"caro/a," like "dear" in English, has two very different meanings. One is expensive and the other is beloved. Because "dear" has this dual meaning, it isn't clear which one is meant when you translate with "dear" instead of "expensive."
The fact that "dear" can mean "expensive" in English is nice, but it doesn't solve the ambiguity. In other words, Duo needs to be sure you mean "expensive" when you translate "caro" in this sentence. If you translate it as "dear," Duo has no way of knowing which of the two meanings you intend. Expensive is a perfectly good British English word. So, there's no reason not to use it.
Sorry, I wrote the following without noticing that you asked about the Spanish, and your question was three years ago (!) I didn't delete it because I thought it might help someone else
In manufacturing, the process of producing stages of a PRODUCT are called a production line; producing items is called production. Outside of manufacturing, one might say, "Incentives put into place last quarter caused the farmers to have a direct increase in the production of food." "There was an decrease in milk production this winter, when the dairy cows' ration of hay was cut back."