Điền từ thích hợp vào chỗ chấm

For over two hundred years, scholars have shown an interest in the way children learn to speak and understand their language. Several small-scale studies were carried out, especially towards the end of the nineteenth century, (1)______________ data recorded in parental diaries. But detailed, systematic investigation did not begin until the middle decades of the twentieth century, when the tape recorder (2) ______________ into routine use. This made it possible to keep a permanent record of samples of child speech, so that analysts could listen repeatedly to obscure (3) ________________, and thus produce a detailed and accurate description. The problems that have (4) ______________ when investigating child speech are quite different from (5) __________________ encountered when working with adults. It is not possible to carry out certain kinds of experiments, because aspects of children’s cognitive development, such as their ability to (6) _________________ attention or to remember instructions, may not be sufficiently advanced. (7)________________ is it easy to get children to (8) __________________ systematic judgments about language – a task that is virtually impossible below the age of three. Moreover, anyone who has tried to make a tape recording of a representative sample of a child’s speech knows how frustrating this can be. Some children, it seems, are innately programmed to (9) ________________ off as soon as they notice a tape recorder (10) ________________ switched on.