As is known to all language learners, newly learnt words will soon be forgotten unless ______.
A：exactly
B：finally
C：frequently
D：gradually
下列关于溶液的叙述，正确的是( )。
Since teenagers have ____ ____these needs, they often question who they are and how they ____ ____ society.
下列地质作用中，属于内力作用表现形式的是（ ）
