This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteriology, Subject name: Microbiology
This test is positive in 
Select and explain. 
A. Streptococcus pyogenes, B. Streptococcus pneumoniae, C. Streptococcus mitis, D. Streptococcus agalactiae
B: Streptococcus pneumoniae
Answer: B: Streptococcus pneumoniae 
Explanation: Test is quellung reaction quellung reaction: a small amount of growth is mixed with saline, antisera against the capsule polysaccharide, and methylene blue stain. After incubation at room temperature for 1 hour, the reaction is observed under the microscope.The organisms are outlined in light blue. A positive reaction shows clumping because of cross-linking of the antibodies and pneumococci. The halo effect around the pneumococci is apparent capsular swelling.Jawetz 7e pg 224