In an Agglutination Test, Eight Serial Dilutions to Determine Antibody

In an agglutination test, eight serial dilutions to determine antibody titer were set up: tube #1 contained a 1:2 dilution; tube #2, a 1:4, etc. If tube #6 is the last tube showing agglutination, what is the antibody titer?

A) 1:32
B) 1:6
C) 32
D) 64
E) 6