A.If the bag was not heavy, we would take it with us.
B.Had the bag not been heavy, we would have taken it with us.
C.If had the bag not been heavy, we would have taken it with us.
D.Unless the bag had not been heavy, we would have taken it with us.

A.a chemical compound  
B.The tip of a root
C.A biochemical process  
D.A type of plant cell

A.a chemical compound  
B.The tip of a root
C.A biochemical process  
D.A type of plant cell

A.The results of two experiments are compared
B.A generalization is made and several examples of it are given
C.A hypothesis is presented, and several means of providing it are suggested
D.The findings of an experiment are explained

A.The relationship between a plant’s roots and its shoots
B.How plants can be grown without roots
C.What can be learned by growing roots in isolation
D.What elements are necessary for the growth of isolation

A.obtain organic compounds from the roots
B.do not require water
C.have begun photosynthesizing
D.“export” protein in the form of amino acids

A.necessary
B.predictable  
C.useful  
D.successful

A.highly-paid
B.unemployed  
C.busy  
D.employed

A.highly-paid
B.unemployed  
C.busy  
D.employed

A.Vietnam exports a lot of rice which grown mainly in the south of the country.
B.Vietnam exports a lot of rice, that is mainly grown in the south of the country.
C.Vietnam, which is grown mainly in the south of the country, exports a lot of rice.
D.Vietnam exports a lot of rice grown mainly in the south of the country.