LONDON (Reuters) -- Oil prices tumbled more than a dollar Wednesday, falling below $68 a barrel and breaking a key support level. U.S. light crude for October delivery fell $1.15 to $67.45 a barrel on the New York Mercantile Exchange. For U.S. crude, the 200-day moving average, considered a funds buy/sell trigger, was effective around $67.50. Prices have fallen by more than $11 from a record of $78.65 hit on Aug. 8 when concerns of supply disruption ran high. Since then, diplomatic efforts to try to resolve Iran's dispute with the West have eased worries of possible disruption of the...