Step 1: Note the key ratio.
The problem tells us that the oxygen released by one single tree can support 2 human beings. This is the main link we need.
Step 2: Write down the target.
We want to support 120 human beings. We must find how many trees give enough oxygen for all of them.
Step 3: Set up the proportion.
If 1 tree supports 2 people, then the number of trees needed equals the number of people divided by 2.
Step 4: Do the division.
\[ \text{Trees} = \frac{120}{2} = 60 \]
Step 5: Sense-check with the acre hint.
The line about an acre supporting 18 people is extra background information. The direct tree-to-people ratio of 1 to 2 is what decides the count, and it gives a clean whole number.
Step 6: Conclude.
So the minimum number of trees needed to support 120 human beings is 60 trees.
\[ \boxed{60 \text{ trees}} \]