Option B, deploying a new service and adding a Cloud Load Balancer instance in front of both services, is not recommended because it would require you to create and manage a separate service for the new version, which would be more complex and less efficient than deploying a new revision.
Option C, using continuous deployment with Cloud Build and substitution variables, is not relevant to this scenario because it involves deploying new versions automatically based on changes to a development branch, rather than manually deploying new revisions with a specific percentage of traffic.
Option D, using Traffic Director, is also not relevant because Traffic Director is used for managing traffic between different services or clusters, rather than between revisions of the same service.