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 from 15 to 18.

We first learn about loving and caring relationships from our families. A family is defined as a domestic group of people with some degree of kinship - whether through blood, marriage, or adoption. Ideally, each child is nurtured, respected, and grows up to care for others and develop strong and healthy relationships. This does not mean that it is always easy to make and keep friends; it just means that we share the goal of having strong relationships. "Family" includes your siblings and parents, as well as relatives who you may not interact with every day, such as your cousins, aunts, uncles, grandparents, and stepparents. These are probably the people you are closest to and with whom you spend the most time. Having healthy relationships with your family members is both important and difficult. Families in the 21st century come in all shapes and sizes: traditional, single parent, blended (more than one family together in the same house), and gay and lesbian parents - just to name a few. No matter the "type" of family you have, there are going to be highs and lows - good times and bad. Many times, however, families become blocked in their relationships by hurt, anger, mistrust, and confusion. These are natural and normal, and few families do not have at least a few experiences with them. Families need to be units of mutual caring and support; they can be sources of lifelong strength for all individuals. 

What is the main idea of the passage?