The number of rigs exploring for oil and natural gas in the U.S. dropped by 11 this week to 420, again reaching an-time low amid depressed energy industry prices. A year ago, 905 rigs were active. Houston oilfield services company Baker Hughes Inc. said Friday 332 rigs sought oil and 87 explored for natural gas. One was listed as miscellaneous. Among major oil- and gas-producing states, New Mexico and Oklahoma each lost three rigs. Kansas, Louisiana, Texas and West Virginia were down two apiece. Colorado declined by one. Alaska, Arkansas, California, North Dakota, Ohio, Pennsylvania, Utah and Wyoming were all unchanged. The U.S. rig count peaked at 4,530 in 1981. The previous low of 488 set in 1999 was eclipsed March 11, and has continued to plunge.