That may depend on the specific school(s) that you are applying for. Generally speaking, if you have received A's in most of your subjects with only one B, you should still have a good chance of receiving an offer. Your academic performance may negatively impact your chances if there are more instances of achieving B grades (i.e. if you receive a B in more than one subject), or one (or more) instances of achieving a C grade, which is likely to reduce your chances of receiving an offer, given the overall competitiveness of the process and the likelihood that most other applicants will also have favourable performance.
Of course, your academic performance is usually one of several factors that are considered as part of your application, including ability tests such as the HAST, as well as any extracurricular activities that you may have been involved in prior to applying to a selective school. If you are required to sit the HAST (or another test), it is important that you also focus on maximising your preparations for that test so that you perform at a standard that is superior to that of other applicants, which will hopefully increase your chances of receiving an offer.
I hope this helps!
