This is more appropriate in my opinion
I want to be a police officer. = 我想做一个警察。 --- But to my ears that would be "I would like to do a police officer." 哈哈哈哈
Though 做 means "to do", it has multiple verb functions. So it can also mean "to be" (in a role).
Yhe hints say just cha should work for police officer, so why does it only accept jing cha?