Given the task definition and input, reply with output. This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

Before the latter half of the 18th century, grain, timber and then coal were Plymouth's main imports. During this time the real source of wealth was from the neighbouring town of Plymouth Dock (renamed in 1824 to Devonport) and the major employer in the entire region was the dockyard. The Three Towns conurbation of Plymouth, Stonehouse and Devonport enjoyed some prosperity during the late 18th and early 19th century and were enriched by a series of neo-classical urban developments designed by London architect John Foulston. Foulston was important for both Devonport and Plymouth and was responsible for several grand public buildings, many now destroyed, including the Athenaeum, the Theatre Royal and Royal Hotel, and much of Union Street.
Along with coal and grain, what was the most important import in 18th century Plymouth?