og132一道经典语法题

cuishiliu

                                                                                                                        Grown from seeds saved during the previous year, heirloom tomatoes appear less appetizing than most oftheir round and red supermarket cousins—they are often green and striped, or have plenty ofbumps and bruises—heirlooms are more flavorful and thus in increasing demand.


红色部分是d选项，错误的。我直接贴上来了

og的解释是说这句话是ungrammatical，为什么？？
咱先不考虑语义，我不觉得这整句话有语法错误吖，有人说这是run－on 因为Grown from seeds saved during the previous year, heirloom tomatoes appear less appetizing than most of their round and red supermarket cousins和heirlooms are more flavorful and thus in increasing demand. 中间没有连接词。但是不是用dash连接的吗
我记得曼哈顿里说过，dash可以做逗号，分号，和冒号。这里dash可以做分号隔开这两个独立的句子吖，为什么会有run on？ og为什么说是ungrammatical？