you buy $300,000 of stock on margin by posting a deposit of $200,000 with your broker. Given that the stock doubles in value and that the initial and maintenance margin ratio are both 0.5, how much money may you now withdraw from the account without triggering a margin call whilst keeping the account open?

A: You may withdraw up to $200k

B: You may withdraw up to $300k

C: You may withdraw up to $400k

D: You may withdraw up to $450k

Please explain the answer in detail, thank you!