keep的過去式是什么

　　He kept us at it all day.

　　他讓我們不停地干了一天。

　　For reasons of security the door is always kept locked.

　　為了保證安全，門總是鎖著的。

　　Dogs must be kept on a lead in the park.

　　狗在公園里必須系著牽狗帶。

　　The bills kept coming in, one after another.

　　賬單紛至沓來。

　　Next year's collection is still being kept under wraps.

　　明年的時裝系列仍在保密之中。