Multivariable Regression Prediction Intervals with R



I have the following regression model: A ~ B + C
Using this model, I need to generate a prediction interval for values of B, given set values for A and C. Does anyone know how I could do this in R?

I need to do this because for the dependant variable A, there are target values, so I need to know what values for B we should target in order to reach those target values of A. The values for C are the same as those used in the fitted model.