Which of the following serves as a starting point for duplicate strands of dna to be copied by pcr

You don't state the list so I will give you how PCR works:

1. Denature the double stranded DNA into single strands by heating it at about 95C for 30 seconds (THIS IS PROBABLY YOUR ANSWER!)
2. Anneal the small primers to the single stranded DNA (this is done at various temperatures depending not the melting temperature of the primers) - maybe around 50-55C
3. DNA polymerase then extends from these primers in the presence of excess dNTPs (a mixture of A,T,G,C). This is at 72-80C usually. We use a thermostable polymerase that can withstand all of these temperatures (even 95C!)...but it works at its own temperature.
4. you cycle through 1-3 many more times and this is how you amplify the DNA section you are interested in.

Ithink it’s around 500 million or something. kinda scary honestly but that’s just my opinion.

Bhope this : )