=>
III/
1. The world famous Ha Long Bay is probably one of the most fabulous natural wonders in Viet Nam.
-> one of the most adj N
2. According to the British Royal Cave Association, at 31.4 km thiss is the longest, dry cave in Asia.
-> the longest, dry cave ~ hang động khô, dài nhất
IV/
1. It is said that those dogs are very fierce./Those dogs are said to be very fierce.
-> Chủ động : People/They say that S Vs/es
-> Bị động : It is said that S Vs/es hoặc S is/am/are said to V 
2. The palace was designed by a French architect.
-> bị động QKĐ was/were V3/ed

III.

1. The world famous Ha long Bay is probably one of the most  fabulous natural wonders in Vietnam

2. According to the British Royal Cave Association, at 31.4 km this is the longest dry cave in Asia.

IV.

1. có 2 dạng bị động:

a/ It is said that those dogs are very fierce.

b/ Those dogs are said to be very fierce.

2. The palace was designed by a French architect.