A、There are not equal numbers of males and females. B、The sample pool is too small. C、The questionnaire survey fails to include

On 15 Feb., 1989, an instant survey was carried out among 18 overseas postgraduate students: 11 students were male and 7 were female. The purpose of the survey was to discover the views of the students on the number of matters of personal concern. The survey was conducted by means of a questionnaire given to the students to complete. There were five questions. The first question concerned favorite color, and the second, favorite number. The next three questions were all concerned with aspects of marriage. Number three looked at the ideal age to get married. Number four examined the qualities looked for in a partner. And number five asked about the ideal number of children. The main findings were as follows. Blue was the most popular color. This was followed by green and purple. There was no real significance in the choice of lucky number. About one third of the students said that they had none. 61% of the students selected the age group 26-30 years as ideal for marriage followed by 21-25 years. In looking at the most important qualities in an ideal partner, someone meant the person to be intelligent, others chose natural, still others indicated attractive and honest. The ideal number of children was two followed by three. It is not easy to reach any definite conclusion based upon such a small sample of students from such widely different backgrounds. However. it is clear that a majority favor 26-30 as the ideal age m get married with an intelligent partner and producing two children.
Questions:
15.Who were involved in this instant survey?
16.Which of the following aspect is NOT focused on by the five questions?
17.What color was chosen as the most popular in the survey?
18.Why is it difficult to reach any definite conclusion from the survey?