图1表示某雄性动物(2n=4)体内细胞正常分裂过程中不同时期细胞内染色体、染色单 体和DNA含量的关系,图2表示不同时期的细胞分裂图像。请分析并完成问题: (1)图1中a~c柱表示染色体的是 ,图2中表示体细胞有丝分裂的是 。 (2)图1中的数量关系由 I 变化为 II 的过程中,细胞内发生的分子水平的变化是 和有关蛋白质的合成。 (3)图1中的数量关系由 II 变化为 III ,相当于图2中的 → 过程。 (4)符合图1中 III 所示数量关系的细胞名称是 ,图2中不存在同源染色体的是 。
【单选题】Which of the following is true of perfectly competitive firms?
A.
For a perfectly competitive firm, as long as the price derived from expanded output exceeds the marginal cost of that output, the expansion of output creates additional economic profits.
B.
Producing at the profit-maximizing output level means that a firm is actually earning economic profits.
C.
A competitive firm earning zero economic profit will be unable to continue in operation over time.
D.
A perfectly competitive firm will operate in the short run only at price levels greater than or equal to average total costs.