Helen didn't know anything about it her father told her .
A:if
B:because
C:after
D:until
____ ____ come at 7:30 this evening?
已知某两种物质在一定条件下能发生化学反应,其反应的微观示意图如下,则下列说法中,正确的是 (说明:一种小球代表一种元素的原子)