单项选择题 Emulsion oils and sodium nitrite are both approved additives, but （） cannot be used if any pipes are galvanized or if soldered joints exist.

单项选择题 Emulsion oils and sodium nitrite are both approved additives, but the latter cannot be used if any pipes are （） or if any soldered joints exit.

单项选择题 Either a low injection pressure or a leakage of piston rings is able to lead （）.