The second left-behind refers to the rural left-behind children being received by their parents in the city. Because the parents are busy with work every day, the children are surrounded by strangers without friends and playmates. Which of the following are left behind? ['There are a large number of migrant workers living in apartment houses in a city. Many people take their children to the city for a summer vacation. Every day, there are children who speak accents everywhere in the nearby kiosks and on the square.', 'In a street vegetable market in a new district, Xiaoying, who was brought into the city by her parents, helped her grandma selling vegetables every day to collect money. When there were no guests, she squatted on the ground and looked at the comics.', 'During the summer vacation, an education department organized a large number of left-behind children to enter the city to participate in summer camp activities. Some children can still meet briefly with their long-awaited parents, and some parents take them to visit city attractions.', 'The Xiaochen couple who came to work in the city brought Xiao Ming, a retarded son, to the side. The civil affairs department arranged him to the rehabilitation center for the disabled. The couple only met Xiaoming once a week.']