Within the context of a moving average, what does the term moving indicate?

Question $5$ options:

a$)$

the window of focus for the average moves forward one$-$month

b$)$

the window of focus for the actual item moves forward one$-$month

c$)$

the window of focus for the cost accounting moves forward two months

d$)$

the window of focus for the forecast inventory moves forward two months