题目

已知点C在线段AB上，线段AC=7cm，BC=5cm，点M、N分别是AC、BC的中点，求MN的长度． 答案：解：如答图.∵AC=7cm，BC=5cm，点M、N分别是AC、BC的中点， ∴MC=AC=3.5（cm），CN=BC=2.5（cm）， 则MN=MC+CN=3.5+2.5=6（cm）．（第12题答图） They moved to England in 2007 and lived in a big house, ________ to the south.        A．the windows of which opened B．the windows of it opened        C．its windows opened  D．the windows of which opening