29. Person A and Person B cooperate to type a document. At the beginning, A types 100 words per minute, and B types 200 words per minute. When they complete half of the total amount together, A's speed triples, and B rests for 5 minutes before continuing to type at the original speed. In the end, when the document is completed, A and B have typed an equal number of words. Therefore, the total number of words in this document is ( ).
A. 3000
B. 6000
C. 12000
D. 18000
【Answer】Solution: The work efficiency ratio of A and B in the first half is $100: 200=1: 2$, to complete half of the total work, the work ratio of A and B is the same as the efficiency ratio, which is 1: 2, A completes $\frac{1}{6}$ of the total work, B completes $\frac{1}{3}$ of the total work, in the second half of the work, A's total amount is twice that of B, the later efficiency ratio of A and B is 3: 2,

This indicates that while B is resting, A completes one unit of work, so A's total amount is 4 units, B's total amount is 2 units, meaning A completes $300 \times 5=1500$ (units) in 5 minutes, later A 4 units and B 2 units, which accounts for half, the total number of units is 12, $1500 \times 12=18000$.

Therefore, the answer is: $D$.