What is the biggest city in the world？ That is， __1__ city has the largest population？ This seems like __2__ question to answer， but actually it isn't.
 It's actually rather difficult to say which cities __3__ 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 __4__ 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 __5__ the population of different cities is this： it is almost impossible to get __6__ about the population of all cities for the same year. For example， we might get an estimate of New York's population in 1979 an __7__ of Mexico City's population in 1981. So we really can't compare the numbers because the information is for __8__ different years. the population of cities changes rather quickly， especially in some cases. For example， the population of Beijing， China may increase by 5% each year， so the population figure will change rather quickly.
 So we can say that these are our two __9__ for comparing the populations of cities： one， it's difficult to determine the __10__ of a city， two， it is difficult to get accurate information.

1.  A. what       B. which      C. where   D. how