Democrat Barack Obama on Tuesday challenged Republican White House rival John McCain for using a heavily guarded trip to a Baghdad market as evidence that Iraq's security is improving. McCain and other members of a congressional delegation toured the Bab al-Sharqi market last week, traveling in armored military vehicles and wearing body armor during their hourlong excursion. ``The idea that the situation in Iraq is improving because it takes a security detail of 100 soldiers, three Black Hawk helicopters and a couple of Apache gunships to walk through a market in the middle of Baghdad is simply not credible and...