仔细观察下面的规律。如果这个规律持续下去，第 $99^{\text{th}}$ 个字母是什么？ $\begin{matrix} \text{S} & \text{Q} & \text{U} & \text{A} & \text{R} & \text{E} & \text{S} & \text{Q} & \text{U} & \text{A} & \text{R} & \text{E} & \text{S} & \text{Q} & \text{U} & \text{A} & \text{R} & \text{E} & \dots \\ 1^{\text{st}} & & & & & & & & & & & & & & & & & 18^{\text{th}} \end{matrix}$ 答：___

1. Identify the repeating pattern and its length

The pattern is 'S Q U A R E', which repeats every 6 letters.

2. Find the position of the 99th letter within the cycle

$99 \div 6 = 16$ with a remainder of $3$.

3. Determine the letter corresponding to the remainder

A remainder of 3 means the $99^{\text{th}}$ letter is the $3^{\text{rd}}$ letter in the pattern 'S Q U A R E', which is 'U'.