The world's population took __ to reach the second billion mark. a 300 years b 100 years c 120 years d 200 years

## Step 1<br />The problem is asking for the time it took for the world's population to reach the second billion mark. This is a historical fact-based question.<br />## Step 2<br />According to historical records, the world's population reached the second billion mark approximately 120 years after the first billion was reached.<br />## Step 3<br />The first billion was reached around 1804, and the second billion was reached around 1927.