Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_15187735_18 (segment_d VARCHAR, segment_b VARCHAR)
### Question:
Name the number of sement d for solar water heaters 
### Answer:
SELECT COUNT(segment_d) FROM table_15187735_18 WHERE segment_b = "Solar Water Heaters"