## PT18 S2 Q20 help please.

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gerbal

Posts: 84
Joined: Sun May 30, 2010 12:42 am

### PT18 S2 Q20 help please.

I couldnt decide between A and C for this one. In the passage it says that in a rainfall, a higher proportion of H20 with O2-18 than H20 with ordinary O2 descends to earth. I realize proportion doesnt equal absolute amount, but since the passage compares O2-18 to ordinary-O2 and says O2-18 is higher proportionally, I don't see how C is wrong. Thanks in advance.

reallyemo

Posts: 5
Joined: Sun Jul 11, 2010 7:59 pm

### Re: PT18 S2 Q20 help please.

It's a higher proportion of the oxygen-18 that falls compared to how much of it is in the cloud, not compared to the normal oxygen. So say 1/5 of the cloud is oxygen-18 and then the other 4/5 is normal oxygen. If all of that 1/5 of oxygen-18 fell and 2/5 of the normal oxygen fell, then the amount of the normal oxygen that fell is greater than the amount of oxygen-18 that fell, but proportionally compared to how much was in the cloud in the first place, more oxygen-18 fell than normal oxygen. So you don't necessarily know that the rainfall contains more oxygen-18 than ordinary oxygen. But A is supported in the passage by the second sentence (which says that in a rain cloud, water molecules containing oxygen-18 are rarer than water molecules containing normal oxygen). Does that make sense?

gerbal

Posts: 84
Joined: Sun May 30, 2010 12:42 am

### Re: PT18 S2 Q20 help please.

it says a higher proportion of oxygen-18 than ordinary oxygen, so isn't it comparing oxygen-18 to ordinary oxygen? I do agree that I could eliminate C if I read it your way, but is that the only way to read the sentence? Thanks.

reallyemo

Posts: 5
Joined: Sun Jul 11, 2010 7:59 pm

### Re: PT18 S2 Q20 help please.

it says, in rainfall, a higher proportion of all water molecules containing oxygen-18 than (the proportion) of all water molecules containing ordinary oxygen descends to earth. It is kind of confusing, and I see where you are coming from, but the "than" is comparing the proportions of each to how much was in the cloud previously, rather than comparing the proportions of the molecules fallen to the total of both. So the one proportion is the amount of water molecules containing oxygen-18 that fall compared to all the water molecules containing oxygen-18 in the cloud originally. The stimulus says that this proportion is greater (higher) than the proportion of the water molecules containing ordinary oxygen compared to all the water molecules containing ordinary oxygen in the cloud originally.