Why this query is fast

Welcome to another database question. In this question I created a community poll question and provided some answers. All answers can be correct of course but the question is what is the most efficient? this is what I try to explore in this video and compare how different database platforms such as mysql or Postgres differ in those implementations; Table T with three integer fields A, B and C. A has a primary key clustered index. B has a secondary index. Which query is more likely to be the most efficient? 0:00 Intro 1:00 The Question 3:20 WHERE C BETWEEN 50,50000 5:30 WHERE B BETWEEN 50,50000 13:20 WHERE C BETWEEN 50,50000 Fundamentals of Networking for Effective Backends udemy course (link redirects to udemy with coupon) https://network.husseinnasser.com Fundamentals of Database Engineering udemy course (link redirects to udemy with coupon) https://database.husseinnasser.com