Which of the following is true?

a. Those born during winter and spring are less likely to develop schizophrenia later in life.

b. People born in densely populated areas are less likely to develop schizophrenia later in life.

c. Fetuses exposed to flu virus are more likely to develop schizophrenia later in life.

d. Maternal influenza during pregnancy does not affect brain development in monkeys.

e. The retrovirus HERV is found more often in people who do not develop schizophrenia.