先锋部队在主力到达前已经占领了战略要地。

The ___vanguards captured the strategic position before the main forces arrived.

环保组织处于可持续发展运动的最前沿。

Environmental organizations are at the ___vanguard of the sustainability movement.

这些科学家是人工智能领域的先锋。

These scientists are the ___vanguards in the field of artificial intelligence.