How to find time complexity of an algorithm - Interview Questions

How to find time complexity of an algorithm

There are 2 buildings and on each’s window, a flower pot is kept. Ravi’s mother tells him to multiply each cell/window to the other and store in a matrix ? What would be time complexity if he writes a code to do so ?

Answer : D. Theta(n2)

A code with θ(n) and θ(n2) complexity. Which code will execute faster for a code of size J ?

Answer : C. Cant be said as size of K is unknown