Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

By adopting a few simple techniques, parents who read to their children can substantially increase their children’s language development. It’s surprising, but true. How parents talk to their children makes a big difference in the children’s language development. If a parent encourages the child to actively respond to what the parent is reading, the child’s language skills increase.

A study was done with 2 to 3 year-old children and their parents. Half of the thirty children participated in the experimental study; the other half acted as the control group. In the experimental group, the parents were given a two-hour training session in which they were taught to ask open-ended questions rather than yes/no questions. For example, the parent should ask, “what is the doggie doing?” rather than, “Is the doggie running away?” Experimental parents were also instructed in how to expand on their children’s answers, how to suggest alternative possibilities, and how to praise correct answers.

At the beginning of the study, the children did not differ on measures of language development, but at the end of one month the children in the experimental group tested 5.5 months ahead of the control group on a test of verbal expression and vocabulary. Nine months later, the children in the experimental group still showed an advance of 6 months over the children in the control group.

The author uses the word measures in line 12 to refer to which of the following?

Certain courses are compulsory; others are optional. voluntary