Except as provided in subsection (b) of this section, all assets of an employee benefit plan shall be held in trust by one or more trustees. Such trustee or trustees shall be either named in the trust instrument or in the plan instrument described in section
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(a) of this title or appointed by a person who is a named fiduciary, and upon acceptance of being named or appointed, the trustee or trustees shall have exclusive authority and discretion to manage and control the assets of the plan, except to the extent that—