SA node normally fires at a rate of about:
100 beats/min (C) is the right choice. This sits at the heart of Cardiovascular System: Heart: the wording points straight to what is happening in the tissue or organ system. A useful study habit is to ask what the structure or process is for: 100 beats/min is the option that correctly names the structure, process, or principle asked for in Cardiovascular System: Heart. That reasoning supports 100 beats/min. One solid sentence about why this works is enough for recall — you do not need a long essay for every MCQ. When you practise, cover the choices first, write 100 beats/min from memory, then reveal the letter — that habit builds real recall for Cardiovascular System: Heart.
Ref: Animal physiology is Unit 7 of the CSIR NET Life Science Syllabus, covering core body systems. It accounts for roughly 10% of the total marks in Sections B and C.