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“Rib” means to work in whatever rib pattern you have been using, knit 2, purl 2 or knit 1, purl 1.
There are often directions similar to these at the transition in a sweater between the ribbing and the main body of the sweater.
Work across in your rib pattern and place the increases after about one-third and two thirds of the sts have been worked. The increases will disrupt a rib but don’t let them affect the overall pattern of ribbing.