Fill in the Blanks (Reading & Writing)

Pearl farming involves inserting a small irritant into an oyster, which then —secretessecretesecretingsecreted layers of nacre around it over several years. Farmers must monitor water quality closely, since polluted conditions can —killkillingkilledkills the oysters before harvest. The resulting pearls are graded by lustre, shape and —sizesizeablesizingsized, all of which determine their market value.