Select the letter-cluster that can replace the question mark in the given letter-cluster series.

AE, KQ, EI, LR, IO, ?, OU, NT

MS

The given letter-cluster series alternates between the vowels "AEIOU" and the consonants "KLMNR." The pattern is as follows:

AE

KQ

EI

LR

IO

Following this pattern, the next letter-cluster should be a consonant. Among the provided options:

NR: (vowel)

MR: (vowel)

NS: (vowel)

MS: (consonant)