Here's why:

* Ribosomes are the protein synthesis machinery of the cell. They read the genetic code from messenger RNA (mRNA) and use that information to assemble amino acids into proteins.

* rRNA is a structural component of ribosomes. It provides the framework for the ribosome's shape and helps to position the mRNA and transfer RNA (tRNA) molecules during protein synthesis.

* There are different types of rRNA: In eukaryotes, there are three main types of rRNA (18S, 5.8S, and 28S rRNA) that are essential for ribosome function.

So, to answer your question directly, rRNA is the specific type of RNA found within ribosomes.