Comprehend key vocabulary:Saira speaks so fast that it's _ to understand her sometimes.

Explanation

Option A: Saira speaks so fast thus it doesn't make sense that it's easy to understand her.Option B: This is correct as it makes perfect sense that its difficult to understand Saira because she speaks so fast.Option C: This is incorrect as slow doesn't fit in this statement.Option D: This is incorrect because wrong doesn't fit in this statement.