Idon't understand this and i need

When you have a row of dominoes set up, you knock over the first one, and what happens to the last one is the certainty that the last one will go over very quickly.

That was the U.S. fear about Communism. If one country became communist, the neighboring country would fall to communism just as the dominoes would fall.

abu bakr, was a senior companion and—through his daughter aisha—a father-in-law of the islamic prophet muhammad. abu bakr became the first openly declared muslim outside muhammad's family. abu bakr served as a trusted advisor to muhammad.

i think it is the third one.