GMAT Sentence Correction (SC) | Re: In order to ensure that chemical reaction in the lab behave

If you use "instead of", the parallelsim must be between two noun phrases (as you correctly identified):

1. the total surface area of the flask
AND
2. a series of irregular points on the surface

The preposition "over" then covers both the above noun phrases. However the usage "OVER a series of points..." is idiomatically incorrect. The correct usage is "TO a series of points....." (Correct usage:over a surface, to a point ). Thus the parallelsim should