Does anyone know how many oxygen sensors a 1999 ML-320 have?

According to Auto-Haus there are four but I'd like to double check.

Additionally, these sensors appear quite pricey. Does anyone know of a better source?

I attached a pic which I took from Auto Haus website related to this issue.

I look forward to your replies.

Thanks!

[IMG]local://upfiles/19645/72E762A38F94457B9B7798FAACBBFF22.jpg[/IMG]

 

yes 4 of them

 

How do you know thatall of themneed to be replaced? I have 150K miles on my 99 ML320 with original sensors w/out any problems.