The phrase, to be strained, could best be replaced by which of the following?

The sugar maple is a hard maple tree. It can grow as tall as 100 feet and as wide as four feet. The sugar maple is commercially valued for its sap, which is used in making maple syrup. The two north-eastern states, Vermont and New York, are major producers of maple syrup. In Canada, Quebec's annual syrup production surpasses 2.5 million gallons. To make pure maple syrup, holes are made in the trunk of the tree at the end of winter or in early spring. The water-like sap seeps through the holes and runs through a plastic spout that is put into the hole. Afterwards, the collected sap is transferred into tubes that are hooked up to a tank kept in the sugar house. Then, the sap goes through a boiling process. Boiling enhances the flavour and adds colour to the sap. Once the sugar content of the sap is about 65-66 per cent, the sap is ready to be strained and marketed. However, maple syrups found in supermarkets are usually not pure and have other additives. The colour of pure maple may range from golden honey to light brown. Between 35 to 50 gallons of sap is needed to produce 1 gallon of maple syrup. Also popular for the strength and finish of its wood, the sugar maple tree has been put to use in furniture, interior woodwork, flooring, and crates.