Student A and Student D have correctly simplified the expression. The expression 154 can be written as 3h + 21 + 15-4h+6+h, which is the same as 21 - 3h + 3h + 21. Student A and Student D both simplified the expression to 21 - 3h. Student B simplified the expression to 18h and Student C simplified the expression to 600p, which are both incorrect.