Oil dropped by $2 a barrel on Tuesday after two sessions of gains as jitters about the global economic outlook and a firmer dollar countered optimism about demand in China and expectations of a drop in US crude inventories. Brent crude fell by $1.93, or 2.4 per cent, to $80.80 a barrel by 11:30 a.m. ET (1530 GMT). US West Texas Intermediate crude dropped $1.73 to $77.02. On Monday, both contrac