The heading that covers the whole paragraph

Early rooftop farms used shallow trays to keep roofs light. This limited the crops they could grow. New lightweight growing materials now allow deeper beds without a large rise in roof load.

Choose the correct heading for the paragraph.

A new material removes a practical limitRight.

A new material removes a limit. It covers both the problem and the solution.

Why rooftop farms used shallow traysNot this one. That is the opening detail only.The weight problem of roof gardensNot this one. It covers the problem but not the solution.Crops that grow best on roofsNot this one. No crops are named or compared.

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