All of the guests         by 9 o’clock , but the host        until 15 minutes later .

A．arrived ; didn’t turn up

B．had arrived ; didn’t turn up

C．arrived ; hadn’t turned up

D．had arrived; hadn’t turned up

下列哪一组在维持血小板的形态、释放和收缩中起重要作用（）

A.骨架系统和收缩蛋白

B.致密颗粒，α-颗粒和溶酶体颗粒

C.ADP，β血小板球蛋白

D.ATP和纤维蛋白

E.微管，微丝和膜下细胞丝