Hi Frank,
For this time I don't recommend you to take time series. You can work with B, C and D variables in a multiple regression model. Try to do a model and post the result here.
I give you and idea, you can transform date into a new variable. For example, in a 52 weeks year, week 1 to week 8: Good production season, week 9 to week 20: Bad production season, week 21 to week 52: normal production season. Now, you will have a new variable:
For example, variable "E" , the factors of this variable could be: Good production season, bad production season and normal production season. Replace their names with just: "BAD", "NORMAL" and "GOOD" and done. You have a new column variable that will make your model more efficient.
I remain attentive to your comments.