Population growth rates vary among regions and even among countries within the same region.
Đáp án đúng là: D. fluctuate
Giải thích:
The sentence describes how population growth rates change or differ across regions and countries. The word 'fluctuate' means to rise and fall irregularly in number or amount, which fits the context of varying rates. 'Unchange' means not changing, which is the opposite of what the sentence implies.
