In this task, you are given a question and a context passage. You have to answer the question based on the given passage.

What moved on keels?, Context: In the Sandgate area, to the east of the city and beside the river, resided the close-knit community of keelmen and their families. They were so called because they worked on the keels, boats that were used to transfer coal from the river banks to the waiting colliers, for export to London and elsewhere. In the 1630s about 7,000 out of 20,000 inhabitants of Newcastle died of plague, more than one-third of the population. Specifically within the year 1636, it is roughly estimated with evidence held by the Society of Antiquaries that 47% of the then population of Newcastle died from the epidemic; this may also have been the most devastating loss in any British City in this period.
coal