the members of which engage, or have engaged within the past 5 years, in a continuing series of offenses described in subsection (c); and

that has as 1 of its primary purposes the commission of 1 or more of the criminal offenses described in subsection (c);

(b)

Penalty.—

The sentence of a person convicted of an offense described in subsection (c) shall be increased by up to 10 years if the offense is committed under the circumstances described in subsection (d).