What are "acid tests" used for? $$ a$.$ To compare two variables by presenting those variables to a random "split" audience over a specific period of time in a controlled and unbiased manner. $$ b$.$ To examine the business' understanding of the segment needs and ability to create value proposition. $$ c$.$ Is a method that focuses on systematically increasing the percentage of website visitors who take desired action on a given page. $$ d$.$ To generate demand with data$-$driven technology.