将大肠杆菌放在¹⁵N培养基中培养若干代后，大肠杆菌DNA中所有的氮均为¹⁵N。然后将被¹⁵N标记的大肠杆菌DNA，转放到¹⁴N培养基中，连续培养三代，在第三代大肠杆菌DNA总量中带有¹⁵N标记的DNA占[ ]

A．12.5%

B．25%

C．5%

D．75%

She saw an advertisement for volunteers at her local gym and as she is interested in the whole area of

diet and exercise, she thought she would ________. [    ]

A. help out  

B. hang out

C. try out  

D. hand out