正文中讨论生命周期假说时，假设收入在退休前的时期中是不变的。然而，对大多数人来说，一生中收入是增长的。在下列条件下，收入的这种增长如何影响图16-22中所表示的一生消费与财富积累的模式? a．消费者可以借贷，因此，他们的财富可能是负的。 b．消费者面临防止其财富降至零以下的借贷制约。你认为(a)和(b)哪个更为现实?为什么? In the discussion of the life-cycle hypothesis in the text, income is assumed to be constant during the period before retirement. For most people, however, income grows over their lifetimes. How does this growth in income influence the lifetime pattern of consumption and wealth accumulation shown in Figure 16-22 under the following conditions? a．Consumers can borrow, so their wealth can be negative. b．Consumers face borrowing constraints that prevent their wealth from falling below zero. Do you consider case (a) or case (b) to be more realistic? Why?