How many strings of eight English letters are there: a) that contain no vowels, if letters can be repeated? b) that contain no vowels, if letters cannot be repeated? c) that start with a vowel, if letters can be repeated? d) that start with a vowel, if letters cannot be repeated?

a) that contain no vowels, if letters can be repeated

Explanation:

since strings of eight English letters contain no vowels, so there are 21 choices for each of the eight letters.

the aesthetic impact is that moment when you are relating spatially to the target. the "ai" occurs when you have an impression that includes both you and the target and is usually a spatial relationship.