Inbreeding for five generations led to production of homozygous transgenic mice.However, these homozygous males or females were infertile. Which of the following approaches is most preferable and economical to obtain heterozygous transgenic animals continuously ?

When domestic sewage mixes with river water (a) small animals like rats will die after drinking river water (b) the increased microbial activity releases micronutrients such as iron (c) the increased microbial activity uses up dissolved oxygen (d) the river water is still suitable for drinking as impurities are only about 0.1 %