1) based on the text What does the text imply is the positive return from "investing" in accounts receivables?

As is true of other current assets, accounts receivable should be thought of as an investment. The level of accounts receivable should not be judged too high or too low based on historical standards of industry norms, but rather the test should be whether the level of return we are able to earn from this asset equals or exceeds the potential gain from other investments. For example, if we allow our customers five extra days to clear their accounts, our accounts receivable balance will increase—draining funds from marketable securities and perhaps drawing down the inventory level. We must ask whether we are optimizing our return, in light of appropriate risk and liquidity considerations

2) Of the four explicit carrying costs, which wouldn't be linear?  Also explain how that one could be linear over specific order size ranges.

Carrying costs include interest on funds tied up in inventory and the costs of warehouse space, insurance premiums, and material handling expenses. There is also an implicit cost associated with the dangers of obsolescence or perishability and rapid price change. The larger the order we place, the greater the average inventory we will have on hand, and the higher the carrying costs.