----people's dress differs from country to country is a refl

正确的答案是
B.that
(That) people's dress differs from country to country is a reflection of cultural differences
完整的句子是
The fact that people's dress differs from country to country is a reflection of cultural differences
“The fact ” 被省略了.
希望帮到了你,
我是加拿大人,前英语老师.