White blood cells, part of the system, work closely with the system to protect us from infection and disease.
a)
circulatory; immune
b)
endocrine; lympathic
c)
circulatory; excretory
eliminate
d)
circulatory; respiratory

The answer is A)
circulatory; immune

The scientific method is a series of steps followed by scientific investigators to answer specific questions about the natural world. it involves making observations, formulating a hypothesis, and conducting scientific experiments.the steps of the scientific method are •observation-the first step of the scientific method involves making an observation about something that interests you•question-once you've made your observation, you must formulate a question about what you have observed. •hypothesis-once you've made your observation, you must formulate a question about what you have observed. •experiment-once you've developed a hypothesis, you must design and conduct an experiment that will test it.•results-the results are where you report what happened in the experiment. •conclusion-the final step of the scientific method is developing a conclusion.i hope that this you : )

Because they are stupid and insensitive