Given an initially empty Doubly Linked List and the following operations:Step $1$:Push back $40$Step $2$:Push front $20$Step $3$:Erase before $40$Step $4$:Push front $30$Step $5$:Push back $10$Step $6$:Insert $50$ before $30$ Type the value of each item in the list separated by a comma $($e$.$g$\mathrm{.}10$ or $10,20,30)$a$.$What is the content from head to tail after Step $2?$Question Blank $1$ of $3$b$.$What is the content from head to tail after Step $4?$Question Blank $2$ of $3$c$.$What is the content from head to tail after Step $6?$