Twins have a unique connection to each other, and they often have a shared appearance. Here are some interesting facts about them. There are two types of twins: fraternal and identical. Fraternal twins __39__ two separate eggs in the womb. They often don’t look similar, and they can be sister and brother. Identical twins are formed when a single egg __40__ into two. Because of this, identical twins usually look alike and are always either sisters or brothers. Although twins often look similar, they never have the same fingerprints. That’s because fingerprints are not completely determined by genes. Similarly, no two people, __41__ twins, have the same belly buttons. They get their shape when the end of the cut umbilical __42__ falls off. The birth rate for fraternal twins has increased over the last four decades, partly because older mothers are more likely to have twins. __43__, more women are using infertility treatments. This increases the chance of having twins.
【題組】39.
(A) put on
(B) develop from
(C) are steeped in
(D) carry out