Here's why:
* Person is always singular, even when referring to a large number of people.
* Thousands is a plural noun, indicating a large quantity.
* Tons is a plural noun, referring to a unit of weight.
* Rice is a singular noun, but in this context, it's being used to describe a large quantity of rice.
Therefore, the phrase "thousands of tons of rice" describes a large amount of rice, and it is plural.