Which of the following might be considered a viable application of a radioactive tracer? It can be more then one?

Aadding a stream of radioisotopes to waste water to examine where pollution goes when it flows into the

ocean

Bembedding a radioactive plug in a valuable necklace so that its location can be determined using a Geiger

counter

Cplacing radioactive sources around a room to trace the location of people as they move through the room

d.) filling a pipeline with radioactive gas to see if it contains any leaks