Pick can be a transitive verb or an intransitive verb, meaning to choose and choose. Picking up means picking up, picking up and choosing. Pick can be used as an intransitive verb without adding objects and adverbials. Transitive verbs must have objects, but adverbials cannot. For example, you didn't pick it. This is the order of the supervisor. It's not your choice. This is the order of the supervisor. You'd better choose one. You should pick up the rubbish you just threw on the ground.