根据短文内容,选词填空。 As we know, the world has a population of 6.8 {#blank#}1{#/blank#} (billion/billions) and it is increasing {#blank#}2{#/blank#} (by/to) 80 million every year. China has already {#blank#}3{#/blank#} (reach/reached) 1.3 billion, so it has the largest population in the world. It's reported that the population in developing countries {#blank#}4{#/blank#} (is/are) larger and is growing faster than {#blank#}5{#/blank#} (those/that) in developed countries.
As a result, the population problem has become more serious in {#blank#}6{#/blank#} (developing/developed) countries.