This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
Isotope (s) used in high dose rate brachytherapy – a) lr192b) Co–60c) Cs–l33d) Ra 226 e) Pd103 
Select and explain. 
A. ac, B. a, C. ab, D. bc
C: ab
Answer: C: ab 
Explanation: "Although cobalt - 60 and cesium - 137 have been used in the past, iridium - 192 is the most commonly used radioisotope in HDR brachytherapy".                                                                       — The physics of radiation therapy
Note - Cs-137 is used in HDR brachytherapy, while option c i.e. Cs - 133 (not Cs - 137).