If there is no vaccine against HIV, what is the possible reason?

Explanation

Option A is correct since HIV can be mutated very rapidly and numerous strains of HIV exist. The mechanism behind such mutations is thought to be due to the inability of the viral reverse transcriptase to accurately transcribe RNA to DNA. Indeed, the rates of these spontaneous mutations determine the genetic diversity and evolution of RNA viruses. Option B is incorrect since the HIV vaccine if made, will prove to be cost-effective as per the research conducted. Option C is incorrect since the spread of the virus can be reduced by better hygiene, for example, sterilization of syringes and razors. But this method is difficult to achieve in low-income countries for mass protection against the virus. Option D is incorrect since it states "none of these".