each 后面的名词要用单数

当时的问题出在 problem 被写成了复数：

❌ each problems
✅ each problem

each 虽然用来指不止一个事物，但后面的名词必须用单数。例如：

❌ Each people must complete the test alone.
✅ Each person must complete the test alone.
❌ We delivered a card to each houses in the neighbourhood.
✅ We delivered a card to each house in the neighbourhood.