He learned the rules of the game _______.

选择一项：
a. one by one
b. step by step 解析：考查固定短语。句意：他逐渐学会这个游戏的规则。one by one意为“一个接一个”；step by step意为“一步步；逐渐”；side by side意为“肩并肩”；shoulder to shoulder意为“肩并肩”。根据句意，。
c. side by side
d. shoulder to shoulder