An analyst does research about price multiples. For a given company, price to earnings ratios are generally more stable from period to period than:

A. price to sales ratios but not price to cash flow ratios.
B. price to cash flow ratios but not price to sale ratios.
C. neither price to sales ratios nor price to cash flow ratios.

空调器制冷时叶片应向上，制热时叶片应向下，以利用室内空气有效对流，提高制冷或制热效率，减少耗电量。