How does exercise sharpen the mind according to Para.2?
A、By increasing the brain siz
B、By decreasing the protein level.
C、By extending the time of concentration.
D、By strengthening brain cells’ connections.
The old man's clothes, ____ old and worn, looked clean and of good quality.
A、if
B、when
C、though
D、since
4.In each of the following groups of words, there are four underlined letters or letter combinations marked A, B, C and D. Compare the underlined parts and identify the one that is different from the others in pronunciation.