Questions 18 to 20 are based on the following passage.At the end of the passage, you will be given 15 seconds to answer the questions. Now, listen to the passage.
A、long-term study of 136 pairs of twins showed that ______ .A．they lost a little ability to reason over the years B．the only factor which declined over the years was their speed with which to perform mental tasks C．their memory was not as good as it had once been D．their minds became a bit more active as they grew older
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