Kinetics of High-Temperature Short-Time UHT processing:
Assertion (A) is correct: Ultra-High Temperature (UHT) processing ($135\text{--}150^\circ\text{C}$ for 2 to 5 seconds) accomplishes complete commercial sterility with extensive bacterial spore destruction while minimizing chemical damage, browning (Maillard reactions), cooked flavor, and nutrient loss.
Reason (R) is correct: The temperature coefficient ($Q_{10}$, the factor by which reaction rate increases for a $10^\circ\text{C}$ rise in temperature) for bacterial spore inactivation is exceptionally high ($Q_{10} = 8\text{ to }30$), whereas the $Q_{10}$ for undesirable chemical reactions (Maillard browning, whey protein denaturation, thiamine degradation) is only $1.5\text{ to }2.0$. Consequently, increasing temperature while dramatically shortening holding time exponentially increases spore lethality while leaving chemical degradation virtually unaffected.
Because the disparity in $Q_{10}$ values described in (R) directly explains why spore kill is maximized over chemical changes in (A), both (A) and (R) are correct and (R) is the correct explanation of (A).