一道关于宏观经济的英文选择题The technique used to calculate the CPI implic

一道关于宏观经济的英文选择题
The technique used to calculate the CPI implicitly assumes that consumes by:
a.relatively less of goods with relative prices that are decreasing
b.goods and services whose quality improves at the rate of growth of real GDP
c.the same relative quantities of goods as in a base year.
但不太清楚为什么有高人能帮我讲解一下嘛,