In an agglutination test, eight serial dilutions to determine antibody titer were set up:
Tube 1 contained a 1:2 dilution;
Tube 2 contained a 1:4 dilution, etc.
If tube 6 is the last tube showing agglutination, what is the antibody titer?
a) 64
b) 32
c) 128
d) 256