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X plays a vital / critical / pivotal / important role in...
X is fast becoming a key instrument in ...
X is essential for ...
X is known as the most ...
X is an significant component in ... and plays a key role in ...
In..., X has been thought of as a key factor in
In..., X has become a central issue for ...
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X is a common condition which has considerable impact on ...
X is one of the most widely / extensively used groups of ...
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A key aspect of X is ...
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X is a classic problem in ...
A primary concern of X is ...
X is a dominant feature / property of ...
X is an important aspect of ...
The concept of X is central to ...
X is at the heart of our understanding of ...
X has been instrumental in our understanding of ...
Investigating X is a continuing concern within ...
X is a major area of interest within the field of ...
The theory of X provides a useful / instrumental account of how ...