Answer the following question given this paragraph: 

Each muscle fiber contains hundreds of organelles called myofibrils. Each myofibril is made up of two types of protein filaments: actin filaments, which are thinner, and myosin filaments, which are thicker. Actin filaments are anchored to structures called Z lines (see Figure below ). The region between two Z lines is called a sarcomere. Within a sarcomere, myosin filaments overlap the actin filaments. The myosin filaments have tiny structures called cross bridges that can attach to actin filaments.


Q: Each muscle fiber contains hundreds of organelles called what?

 Choices:

- fast glycolytic fibres

- slow twitch fibres

- pores

- myofibrils

A:
myofibrils
