Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
In multicellular organisms, specialized cells may be organized into tissues. Tissues may be organized into organs, and organs may be organized into organ systems. Organ systems work together to carry out all the functions of the whole organism.


Question: In multicellular organisms, specialized cells may be organized into what, which in turn may be organized into organs?
tissues
Tissues.