Final answer:
Increasing the temperature of the annealing phase may reduce precision and yield due to inefficient primer binding, while extending the elongation phase may increase yield if the enzyme has more time to synthesize DNA but could reduce precision if too lengthy.
Step-by-step explanation:
Adjustments that increase the temperature of the annealing phase and the length of the elongation phase in a PCR reaction can have significant impacts. If the annealing temperature is too high, as in the case of an increase from the optimum 50°C to 65°C, the primers may not bind efficiently to the template DNA, which might reduce both the precision and yield, as non-specific binding or failed annealing occurs. Conversely, extending the elongation phase can allow more time for DNA synthesis, potentially increasing the yield if the enzyme has more time to work, but if too long, it could also introduce errors, reducing the precision of the amplification. The optimal answer is that precision and yield will be reduced (a).