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Pipes are made of three main materials:
1) Metal. This is the strongest material, but it is also the heaviest, and the most rigid. It is also the most expensive of the three materials.
2) Rubber. This is the most flexible of three materials. But it is the weakest.
3) Plastic. This is the lightest material. It is also the least expensive of the three.
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Metal is …
the most flexible of three materials
the weakest of three materials
the least expensive of the three
the most expensive of the three materials