Which of the following is NOT a valid cloud implementation type?.

a$.$ Community cloud, which is a type of cloud built by and for a specific group of organizations that share a common trade

b$.$ Shared cloud, which is a type of cloud built by a group of organizations for the purpose of sharing it with the public

c$.$ Public cloud, which is a type of cloud infrastructure built by a third$-$party organization to sell cloud services to the general public

d$.$ Private cloud, which is a type of internal cloud built by an organization for the sole purpose of servicing its own needs