The correct past tense forms of 'to be' are 'was' for singular subjects and 'were' for plural subjects. 'I' takes 'was' and 'they' should use 'were'. Option C correctly uses 'was' with 'I'. Option A uses 'is' which is present tense. Option B should use 'was' with 'she'. Option D uses 'are' which is present tense and incorrect with the past time reference 'yesterday'. * **Core考点:** 动词'to be'的过去式 * **解题思路:** 识别主语与谓语动词的一致性 * **选项分析:** * A: 错误，'They' 应该使用 'were'，且 'is' 是现在时。 * B: 错误， 'She' 应该使用 'was'。 * C: 正确，'I' 在过去时使用 'was'。 * D: 错误，'We' 在过去时应该使用 'were', 且 'are' 是现在时。 * **易错点:** 过去时动词的形式与主语的匹配。