$1.$ Calculate the amounts missing in the table above and represented by letters $($a$)$ to $($d$)$ taking into account that the undertaking uses the method of sale value at the point of separation to allocate common costs.

$2.$ Is it profitable for the company to further process $($liquefy$)$ all gases? Please give reasons for your answer.

$3.$ Would your answer to question $(2)$ above change if the undertaking used the physical units method to share the common costs? Please give reasons for your answer.