element ui我想做个多选框，为什么每次点击都是全选呢？

`<el-form :model="ruleForm" status-icon :rules="rules" ref="ruleForm" label-width="100px">
<el-form-item label="多选题">
<el-input v-model="ruleForm.multipleChoiceQuestion" placeholder="请输入单选题的题目"
prop="multipleChoiceQuestion"></el-input>
<el-checkbox-group v-model="ruleForm.multipleChoiceAnswer">
<el-checkbox :label="item"
v-for="item in ruleForm.multipleChoiceAnswers"
:key="item"
:value="item">{{item}}

        </el-checkbox>  

</el-checkbox-group> </el-form-item></el-form>`