91. Read the following passage, then answer the question below.

Most plants build sugar through a sequence of reactions known as the Calvin cycle, which begins when an enzyme called rubisco captures carbon dioxide from the air. The enzyme has an awkward flaw: it can also react with oxygen, and when it does, the plant wastes energy and carbon in a process called photorespiration. The problem grows worse in hot, dry weather, because plants close the tiny pores on their leaves to save water, which lets oxygen accumulate inside while carbon dioxide runs low. Under such conditions, photorespiration can consume a substantial part of what the plant has just produced.

Evolution has found a workaround. Plants of the type called C4, which include maize, sugarcane, and many tropical grasses, first capture carbon dioxide in cells near the leaf surface, using an enzyme that, unlike rubisco, ignores oxygen. The captured carbon is then passed as a four-carbon compound to specialized cells deeper in the leaf, where it is released in a concentrated burst around rubisco. Because rubisco is surrounded by carbon dioxide, it has little chance to react with oxygen. The arrangement costs extra energy, so in cool, moist climates C4 plants have no advantage over ordinary C3 plants and are often outcompeted. In warm, sunny places, however, the extra expense is repaid, and C4 plants grow faster while using less water per unit of growth. C4 photosynthesis has evolved independently more than sixty times, which suggests that the solution is easy to reach once conditions favor it.

These facts matter for agriculture. Rice, a C3 crop, feeds billions, and yields are expected to be hurt by rising temperatures. Researchers in several countries are therefore trying to give rice C4 machinery, an ambitious project that requires changing both the leaf's anatomy and its chemistry. Progress has been real but slow, and no one expects a quick result. Even partial success, though, would be notable: a rice plant that wastes less carbon might yield substantially more grain from the same land and water.

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