Sellers’ costs per unit must ______ with the number of units sold.

A．It could allow electronics manufacturers to increase the density of transistors on silicon chips by 100 times and simplify production meanwhile.
B．A mould is pressed against a piece of silicon and a laser pulse is applied for just 20 billionths of a second before it’s done.
C．It could result in more powerful computers and memory chips.
D．It limits advancements in chip size and speed.

A．many middle-class women do not even consider having their children until, they are over thirty years old.
B．career mums are able to go into higher social status compared with housewives.
C．all are endowed with equality by law but things could be different between career women and housewives.
D．most mothers would choose to stay at home to bring up their children.