Task: Question Answering
Input (English): Question: The system in T: 
 Options:  <1> It is present in all muscle cells without exception. <2> It is part of the sarcoplasmic reticulum. <3> It has an ATPase activated by Ca2 +. <4> It is a tubular invagination of the plasma membrane. <5> Releases Ca2 + to the cytosol during recruitment.
Output (English): 4