A:3米/秒
B:4米/秒
C:5米/秒
D:2米/秒
A. What's the matter?
B. It's red and white.
C. Yes, it is! Thank you.
A: ____
B: I can't find my ball.
A: What colour is your ball?
B: ____
A: Look! Is this your ball?
“十月怀胎,一朝分娩”,胎儿和母亲血脉相连.胚胎在母体内发育的场所是____,胎儿通过____、脐带从母体获得营养物质和氧.