由¹⁴CO和¹²CO组成的混合气体与同温、同压下空气（平均相对分子质量为29）的密度相同，则下列说法正确的是

A．混合气体中¹⁴CO和¹²CO的分子数之比为14∶15

B．混合气体中¹⁴CO和¹²CO的物质的量之比为1∶1

C．混合气体中¹⁴CO和¹²CO的质量之比为14∶15

D．混合气体中¹⁴CO和¹²CO的密度之比为1∶1

Bob's grandmother _________ his little sister at home.[ ]

A. looks at

B. looks like

C. looks after

D. has a look