$1.[6]$ a$)$ All programs can be written in terms of three types of control structures:$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_,\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$

and $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_.$

b$)$ The $\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ selection statement is used to execute one action when a condition is true

or a different action when that condition is false.

c$)$ Repeating a set of instructions a specific number of times is called $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ repetition.

d$)$ When it is not known in advance how many times a set of statements will be repeated, a$($n$)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ value can be used to terminate the repetition.