Reading the lines, we can get the message _ they are worried about their son.
A. between; that
B. behind ; that
C. along; what
D. among; which
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(答案→)A. between; that
B. behind ; that
C. along; what
D. among; which
解析：考查固定词组与同位语从句。Read between the lines领悟言外之意; that they are worried about their son. 是同位语从句解释message的内容。That只起引导作用，不充当任何成分。