Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| PRR Class | Builder’s Model | Build date | Total produced | Wheel arrangement | Service | Power output |
| --- | --- | --- | --- | --- | --- | --- |
| BS6 | VO-660 | 1942–1945 | 12.0 | B-B | Switcher | hp (kW) |
| BS6 | DS-4-4-660 | 1942–1945 | 99.0 | B-B | Switcher | hp (kW) |
| BS7 | DS-4-4-750 | 1951 | 24.0 | B-B | Switcher | hp (kW) |
| BS7 | S7 | 1951 | 7.0 | B-B | Switcher | hp (kW) |
| BS10 | VO | 1943–1945 | 8.0 | B-B | Switcher | hp (kW) |
| BS10 | DS-4-4-1000 | 1948–1950 | 137.0 | B-B | Switcher | hp (kW) |
| BS12 | S-12 | 1951–1956 | 98.0 | B-B | Switcher | hp (kW) |
| BF15 | DR-4-4-1500 | 1940–1950 | 34.0 | B-B | Freight | hp (kW) |
| BF16 | RF-16 (A) | 1950–1952 | 72.0 | B-B | Freight | hp (kW) |
| BF16 | RF-16 (B) | 1950–1952 | 30.0 | B-B | Freight | hp (kW) |
| BP60a | DR-12-8-3000 | 1947–1948 | 24.0 | 2-D-D-2 | Pass | hp (kW) |
| BP20 | DR-6-4-2000 | 1948 | 18.0 | A1A-A1A | Pass | hp (kW) |
| BP20 | DR-6-4-2000 | 1948 | 9.0 | A1A-A1A | Pass | hp (kW) |
| BRS10 | DRS-4-4-1000 | 1949–1950 | 6.0 | B-B | Pass | hp (kW) |
| BRS12 | RS-12 | 1951–1954 | 8.0 | B-B | Pass | hp (kW) |
| BRS15 | DRS-4-4-1500 | 1950 | 8.0 | B-B | Pass | hp (kW) |
| BRS16 | AS-16 | 1953–1955 | 28.0 | C-C | Pass | hp (kW) |
| BRS24 | RT-624 | 1951–1954 | 22.0 | C-C | Transfer | hp (kW) |
| LRS25 | T2500 | 1950–1951 | 22.0 | C-C | Transfer | hp (kW) |
Question:
How many total units were built of Model ds-4-4-660 with a b-b wheel arrangement and a PRR Class of bs6?
Answer:
99.0