All this leads to a population in the twenty-first century that is smaller ________a few years ago.
句意：所有这些导致21世纪的人口比几年前（的人口）更少了。此处省略了“the population”，句子的后半部分相当于“…is smaller that the population was a few years ago”。因此，本题的正确答案为A。