There are two types of twins; identical and non-identical twins. Identical twins are formed from a single egg in mother's body which divides to from two separate babies. Identical twins look the same, and are often dressed by their parents in clothes of the same colors. It is often difficult to tell identical twins from each other, even when they are standing side by side. Non-identical twins come into being when the mother produces two separated eggs a the same time, both of which grow to form babies. In this case the twins look like normal brothers and sisters and are easy to tell one from the other. In the 1970s and 1980s a scientist did some research into twins. He invited many pairs of identical twins to university and asked them to take part in a week of tests. He was particularly interested in adopted   twins who had been separated at birth. He would give the twins different kinds of tests to study their speed of thinking, their speech, their memory, the ways they saw and heard different things, and so on. Time and time again he found separated twins who preferred clothes of the same color, used the same kind of shaving soap, wore the same shaped square glasses and the same colored socks. There is a third type of twins, but it is a very unusual one. Twins which are joined together at birth are known in western countries as Siamese twins. It is difficult to tell identical twins apart because    _   .
they are dressed in the same clothes they are dressed in the same color they are very alike they are standing side by side