Decisions with capacity constraints

Dear Tutor, I am trying to solve some exercises to get prepared to my Management Accounting exam but I am struggling with this one. A company manufactures and sells two products (X and Y) both of which utilise the same skilled labour. For the coming period, the supply of skilled labour is limited to 2,000 hours. Data relating to each product are as follows: Product X Y Selling price/unit £20 £40 Variable cost/unit £12 £30 Skilled labour hours/unit 2 4 Maximum demand (units) per period 800 400 In order to maximise profit in the coming period, how many units of each product should the company manufacture and sell? A) 200 units of X and 400 units of Y B) 400 units of X and 300 units of Y C) 600 units of X and 200 units of Y D) 800 units of X and 100 units of Y The solution is D but, actually I answered A. I computed the contribution for both products which is £8 for X (£20-£12) and £10 for Y (£40-£30) and then, since Y has the greatest contribution, I would produce as much as possible (so 400 units) of Y and then I would saturate the remaining labour hours with 200 units of X. Where's my mistake? Thank you.