There are different star systems. Binary star systems consist of two stars. Multiple star systems consist of two or more stars. The meaning of star system is "A small number of stars."
I think it's either A) a group of hundreds of stars, or B) a group of two or more stars. I would assume it's B), but i could be wrong.
Hope this helps!
The disk diffusion test is used to compare the effectiveness of antibiotics. The incubated plate (results) will show bacteria growth except where antibiotic has effects. The more effective the drug, the inhibition zone will be greater. So the most effective antibiotic is the one that has the largest inhibition zone around the test site.