What is the 'biggest city in the world? That is, (21) city has the largest population? This seems like (22) question to answer, but actually it isn't.

It's actually rather difficult to say which cities (23) the largest. There are two reasons for this difficulty. First of all, it isn't easy to determine a city's boundaries, that is, where a city ends. Nowadays, nearly all cities have a large (24) area around them. So when we talk about the population of a city, we often mean the population of the whole metropolitan area around the city. That means that it's difficult to determine what the population of a city is because it's difficult to define what a city is.

The second reason that it is difficult to 25 the population of different cities is this: it is almost impossible to get (26) about the population of all cities for the same year. For example, we might get an estimate of New York's population in 1979 and an (27) of Mexico City's population in 1981. So we real ly can't compare the numbers because the information is for (28) different years. And the population of cities changes rather quickly, especially in some cases. For example, the population of Jakarta, Indonesia, may increase by 5% each year, so the population figure will change rather quickly.

So we can say that these are our two (29) for comparing the populations of cities: one, it's difficult to determine the (30) of a city, and two, it is difficult to get accurate information.

21. A. what

B． which

C． where

D． how