Whenever the administering authority concludes prior to a preliminary determination under subsection (b) of this section, that there is a reasonable basis to believe or suspect that an upstream subsidy is being bestowed, the time period within which a preliminary determination must be made shall be extended to 250 days after the filing of a petition under section
1671a
(b) of this title or initiation of an investigation under section
1671a
(a) of this title (310 days in cases declared extraordinarily complicated under subsection (c) of this section), if the administering authority concludes that such additional time is necessary to make the required determination concerning upstream subsidization.