Final answer:
The best revision for sentence 7 to provide variety is option C: 'Though there are higher and more massive dams today, Hoover Dam set the precedent for modern dam-building technology.' This revision contrasts current dam advancements with the Hoover Dam's historical significance.
Step-by-step explanation:
To revise sentence 7 and add variety to the paragraph while avoiding repetition, the best option is: 'Though there are higher and more massive dams today, Hoover Dam set the precedent for modern dam-building technology.' This option provides a smooth transition and acknowledges the advancements in dam-building since the construction of the Hoover Dam without detracting from its historic significance. It contrasts the past innovation of the Hoover Dam with the developments that have since occurred.
The Hoover Dam's historic relevance and its role in pioneering modern dam-building techniques is emphasized through this revised structure. Despite being surpassed in size, the innovative practices established during its construction continue to influence dam engineering today. Furthermore, the concessive conjunction '{{tags.tag}}' helps to balance the acknowledgement of the dam's contemporary successors with the recognition of its groundbreaking nature during its time. It is a subtle linguistic pivot that maintains the sentence flow within the larger context of the paragraph.