Games : How is the order of which my units attack determined?

Let's say I have several units on the field, all of which have their cooldowns at 0, as shown in this picture of the first tutorial level:

How is the order that my units attack determined?

• Order they were placed? (Earliest goes first?)


• Number of attacks that it has done? (Least goes first?)


• Is it positional? (Top right to bottom left?)


• Is it random? (Nothing we can do about that)


• Or what?