With the Transmission Electron Microscope Skeletal Muscle Fibers Can Be

With the transmission electron microscope skeletal muscle fibers can be seen to contain structures called triads. What do the two lateral components of a triad represent?

A) Attachment sites for thick myofilaments
B) Sites for calcium sequestration and release
C) Sites for impulse conduction into the fiber
D) Sites for ATP production
E) Sites for synthesis of proteins to be secreted outside the cell