Fox has captured its 18th straight Wednesday in key ratings categories behind an expanded hourlong edition of “American Idol” and a strong showing by the season finale of “That ’70s Show,” according to preliminary nationals from Nielsen.

Moving to the 9 o’clock hour, the penultimate results show of “Idol’s” third season averaged a big 9.6 rating/23 share in adults 18-49 and 22.1 million viewers overall; it was down from last week but still easily led its timeslot. Fox also led from 8 to 9 with an hour of “’70s Show” (5.6/17 in adults 18-49, 11.4 million viewers overall).

ABC was second on the night thanks to the two-hour season finale of “The Bachelor” (5.9/14 in adults 18-49, 12.9 million viewers overall), which opposed “Idol” from 9 to 10 and then led the 10 o’clock hour in 18-49 over NBC’s “Law & Order” (6.4/16 in adults 18-49, 19.1 million viewers overall). Despite the second-place 18-49 finish, “Law” finished its season with one of its best scores of the season, prevailing in adults 25-54 and total viewers.

CBS, led by “King of Queens” (2.8/7 in adults 18-49, 8.5 million viewers overall), finished behind the WB on the night in 18-49. Frog was hopping with the series finale of “Angel” (2.8/7 in adults 18-49, 5.4 million viewers overall).

Preliminary 18-49 averages for the night: Fox, 7.6/20; ABC, 4.9/13; NBC, 4.0/10; WB, 2.7/7; CBS, 2.3/6; UPN, 1.1/3.

In total viewers: Fox, 16.8 million; NBC, 12.1 million; ABC, 11.3 million; CBS, 8.5 million; WB, 5.5 million; UPN, 2.7 million.