"Thr" Refers to a Gene That Governs a Cell's Ability

"Thr" refers to a gene that governs a cell's ability to synthesize threonine from inorganic precursors. What does the designation "thr⁺" indicate?

A) that the bacterium is biologically inactive
B) that the bacterium is biologically active
C) that the allele is the mutant allele
D) that the allele is normal (i.e., wild type)