The reason I have to go is ________ if I don't．A. that she will be disappointedB. because she will be disappointedC. on account of her being disappointedD. that she will be disappointing

本题考查表语从句的引导词及形容词的正确用法。关键点在于：

1. 表语从句通常由that引导，尤其在表达原因时，结构为"The reason is that..."；
2. because不能引导表语从句；
3. on account of后需接名词或动名词，不能直接接句子；
4. disappointed（感到失望的）用于描述人，disappointing（令人失望的）用于描述事物。

选项分析

A. that she will be disappointed

• 正确。that引导表语从句，说明原因，符合固定结构"The reason is that..."。disappointed正确描述人的感受。

B. because she will be disappointed

• 错误。because只能引导原因状语从句，不能用于表语从句。

C. on account of her being disappointed

• 错误。on account of后需接名词或动名词，不能直接接句子。

D. that she will be disappointing

• 错误。disappointing表示事物令人失望，但句中主语是she（人），应使用disappointed。