(d) : $Bt$ toxin genes were isolated from Bacillus thuringiensis and incorporated into the several crop plants such as cotton. The choice of genes depends upon the crop and targeted pest, as most $Bt$ toxins are insect­group specific. The toxin is coded by a gene named cry. These are numerous genes. Two cry genes cry $I\ Ac$ and cry $II\ Ab$ have been incorporated in cotton. The genetically modified crop is called $Bt$ cotton as it contains $Bt$ toxin genes against cotton bollworms.